Biologically speaking, under the right set of circumstances, almost any shelled mollusk can produce some kind of pearl, however, most of these molluscan pearls have no luster or iridescence. The great majority of mollusk species produce pearls which are not attractive to look at, and are sometimes not even very durable, such that they usually have no value at all, except perhaps to a scientist, a collector, or as a curiosity. These objects used to be referred to as "calcareous concretions" by some gemologists, even though a malacologist would still consider them to be pearls. Valueless pearls of this type are sometimes found in edible mussels, edible oysters, escargot snails, and so on. Kenneth Scarrat, director of GIA Bangkok, has recently argued for changes to current nomenclature. He argues conch "pearls" should be referred to (and various other types of mollusc pearls) as simply pearls, not "calcareous concretions".[5]
Shell of the Indian volute or bailer shell Melo melo, surrounded by a number of Melo pearls
A few species produce pearls that can be of interest as gemstones. These species include the bailer shell Melo (genus), the giant clam Tridacna, various scallop species, Pen shells Pinna (genus), and abalones. Another example is the conch pearl (sometimes referred to simply as the 'pink pearl'), which is found very rarely growing between the mantle and the shell of the queen conch or pink conch, Strombus gigas, a large sea snail or marine gastropod from the Caribbean Sea. These pearls, which are often pink in color, are a by-product of the conch fishing industry, and the best of them display a shimmering optical effect related to chatoyance known as 'flame structure'.
Somewhat similar gastropod pearls, this time more orange in hue, are (again very rarely) found in the horse conch Pleuroploca gigantea.
Largest known pearl from a giant clam
The largest pearl known, was found in the Philippines in 1934. It is a naturally-occurring, non-nacreous, calcareous concretion from a giant clam. Because it did not grow in a pearl oyster it is not pearly, instead it has a porcellaneous surface. In other words, it is glossy like a china plate. Other pearls from giant clams are known to exist, but this is a particularly large one. The pearl weighs 14 lb (6.4 kg) and was supposedly first discovered by an anonymous Filipino Muslim diver off the island of Palawan in 1934. According to the legend as it is currently told, a Palawan chieftain gave the pearl to Wilbur Dowell Cobb in 1936 as a gift for having saved the life of his son. The pearl had been named the "Pearl of Allah" by the Muslim tribal chief, because it resembled a turbaned head. Another even more elaborate legend says that this object is actually the Pearl of Lao-Tzu, a cultured pearl created with a carved amulet and then supposedly progressively grafted into several giant clams, before supposedly being lost due to a shipwreck in 1745. [6] This legend has been discredited, however because this pearl is indeed the product of a giant clam, Tridacna gigas, which cannot be grafted. The pearl is also a whole pearl, not a mabe pearl, and whole pearl culturing technology is only 100 years old. [7]
Saturday, June 13, 2009
Wednesday, April 1, 2009
Pearls and calcareous concretions from other species
Biologically speaking, under the right set of circumstances, almost any shelled mollusk can produce some kind of pearl, however, most of these molluscan pearls have no luster or iridescence. The great majority of mollusk species produce pearls which are not attractive to look at, and are sometimes not even very durable, such that they usually have no value at all, except perhaps to a scientist, a collector, or as a curiosity. These objects used to be referred to as "calcareous concretions" by some gemologists, even though a malacologist would still consider them to be pearls. Valueless pearls of this type are sometimes found in edible mussels, edible oysters, escargot snails, and so on. Kenneth Scarrat, director of GIA Bangkok, has recently argued for changes to current nomenclature. He argues conch "pearls" should be referred to (and various other types of mollusc pearls) as simply pearls, not "calcareous concretions".[5]
Shell of the Indian volute or bailer shell Melo melo, surrounded by a number of Melo pearls
A few species produce pearls that can be of interest as gemstones. These species include the bailer shell Melo (genus), the giant clam Tridacna, various scallop species, Pen shells Pinna (genus), and abalones. Another example is the conch pearl (sometimes referred to simply as the 'pink pearl'), which is found very rarely growing between the mantle and the shell of the queen conch or pink conch, Strombus gigas, a large sea snail or marine gastropod from the Caribbean Sea. These pearls, which are often pink in color, are a by-product of the conch fishing industry, and the best of them display a shimmering optical effect related to chatoyance known as 'flame structure'.
Somewhat similar gastropod pearls, this time more orange in hue, are (again very rarely) found in the horse conch Pleuroploca gigantea.
Largest known pearl from a giant clam
The largest pearl known, was found in the Philippines in 1934. It is a naturally-occurring, non-nacreous, calcareous concretion from a giant clam. Because it did not grow in a pearl oyster it is not pearly, instead it has a porcellaneous surface. In other words, it is glossy like a china plate. Other pearls from giant clams are known to exist, but this is a particularly large one. The pearl weighs 14 lb (6.4 kg) and was supposedly first discovered by an anonymous Filipino Muslim diver off the island of Palawan in 1934. According to the legend as it is currently told, a Palawan chieftain gave the pearl to Wilbur Dowell Cobb in 1936 as a gift for having saved the life of his son. The pearl had been named the "Pearl of Allah" by the Muslim tribal chief, because it resembled a turbaned head. Another even more elaborate legend says that this object is actually the Pearl of Lao-Tzu, a cultured pearl created with a carved amulet and then supposedly progressively grafted into several giant clams, before supposedly being lost due to a shipwreck in 1745. [6] This legend has been discredited, however because this pearl is indeed the product of a giant clam, Tridacna gigas, which cannot be grafted. The pearl is also a whole pearl, not a mabe pearl, and whole pearl culturing technology is only 100 years old. [7]
Shell of the Indian volute or bailer shell Melo melo, surrounded by a number of Melo pearls
A few species produce pearls that can be of interest as gemstones. These species include the bailer shell Melo (genus), the giant clam Tridacna, various scallop species, Pen shells Pinna (genus), and abalones. Another example is the conch pearl (sometimes referred to simply as the 'pink pearl'), which is found very rarely growing between the mantle and the shell of the queen conch or pink conch, Strombus gigas, a large sea snail or marine gastropod from the Caribbean Sea. These pearls, which are often pink in color, are a by-product of the conch fishing industry, and the best of them display a shimmering optical effect related to chatoyance known as 'flame structure'.
Somewhat similar gastropod pearls, this time more orange in hue, are (again very rarely) found in the horse conch Pleuroploca gigantea.
Largest known pearl from a giant clam
The largest pearl known, was found in the Philippines in 1934. It is a naturally-occurring, non-nacreous, calcareous concretion from a giant clam. Because it did not grow in a pearl oyster it is not pearly, instead it has a porcellaneous surface. In other words, it is glossy like a china plate. Other pearls from giant clams are known to exist, but this is a particularly large one. The pearl weighs 14 lb (6.4 kg) and was supposedly first discovered by an anonymous Filipino Muslim diver off the island of Palawan in 1934. According to the legend as it is currently told, a Palawan chieftain gave the pearl to Wilbur Dowell Cobb in 1936 as a gift for having saved the life of his son. The pearl had been named the "Pearl of Allah" by the Muslim tribal chief, because it resembled a turbaned head. Another even more elaborate legend says that this object is actually the Pearl of Lao-Tzu, a cultured pearl created with a carved amulet and then supposedly progressively grafted into several giant clams, before supposedly being lost due to a shipwreck in 1745. [6] This legend has been discredited, however because this pearl is indeed the product of a giant clam, Tridacna gigas, which cannot be grafted. The pearl is also a whole pearl, not a mabe pearl, and whole pearl culturing technology is only 100 years old. [7]
Different types of cultured pearls, including black pearls
Black pearls, frequently referred to as Black Tahitian Pearls, are highly valued because of their rarity; the culturing process for them dictates a smaller volume output and can never be mass produced. This is due to bad health and/or non-survival of the process, rejection of the nucleus and their sensitivity to changing climatic and ocean conditions. Before the days of cultured pearls, black pearls were rare and highly valued for the simple reason that white pearl oysters rarely produced naturally black pearls, and black pearl oysters rarely produced any natural pearls at all.
Since the development of pearl culture technology, the black pearl oyster found in Tahiti and many other Pacific Island areas has been extensively used for producing cultured pearls. The rarity of the black cultured pearl is now a "comparative" issue. The black cultured pearl is rare when compared to Chinese freshwater cultured pearls, and Japanese and Chinese akoya cultured pearls, and is more valuable than these pearls. However, it is more abundant than the South Sea pearl, which is more valuable than the black cultured pearl. This is simply because the black pearl oyster Pinctada margaritifera is far more abundant than the elusive, rare, and larger south sea pearl oyster - Pinctada maxima, which cannot be found in lagoons, but which must be dived for in a rare number of deep ocean habitats or grown in hatcheries.
Black cultured pearls from the black pearl oyster — Pinctada margaritifera — are not South Sea pearls, although they are often mistakenly described as black South Sea pearls. In the absence of an official definition for the pearl from the black oyster, these pearls are usually referred to as "black Tahitian pearls".
The correct definition of a South Sea pearl — as described by CIBJO and the GIA — is a pearl produced by the Pinctada maxima pearl oyster. South Sea pearls are the color of their host Pinctada maxima oyster — and can be white, silver, pink, gold, cream, and any combination of these basic colors, including overtones of the various colors of the rainbow displayed in the pearl nacre of the oyster shell itself.
Since the development of pearl culture technology, the black pearl oyster found in Tahiti and many other Pacific Island areas has been extensively used for producing cultured pearls. The rarity of the black cultured pearl is now a "comparative" issue. The black cultured pearl is rare when compared to Chinese freshwater cultured pearls, and Japanese and Chinese akoya cultured pearls, and is more valuable than these pearls. However, it is more abundant than the South Sea pearl, which is more valuable than the black cultured pearl. This is simply because the black pearl oyster Pinctada margaritifera is far more abundant than the elusive, rare, and larger south sea pearl oyster - Pinctada maxima, which cannot be found in lagoons, but which must be dived for in a rare number of deep ocean habitats or grown in hatcheries.
Black cultured pearls from the black pearl oyster — Pinctada margaritifera — are not South Sea pearls, although they are often mistakenly described as black South Sea pearls. In the absence of an official definition for the pearl from the black oyster, these pearls are usually referred to as "black Tahitian pearls".
The correct definition of a South Sea pearl — as described by CIBJO and the GIA — is a pearl produced by the Pinctada maxima pearl oyster. South Sea pearls are the color of their host Pinctada maxima oyster — and can be white, silver, pink, gold, cream, and any combination of these basic colors, including overtones of the various colors of the rainbow displayed in the pearl nacre of the oyster shell itself.
Origin of a natural pearl
Previously natural pearls were found in many parts of the world. Present day natural pearling is confined mostly to seas off Bahrain. Australia also has one of the world's last remaining fleets of pearl diving ships. Australian pearl divers dive for south sea pearl oysters to be used in the cultured south sea pearl industry. The catch of pearl oysters is similar to the numbers of oysters taken during the natural pearl days. Hence significant numbers of natural pearls are still found in the Australian Indian Ocean waters from wild oysters. X-Ray examination is required to positively verify natural pearls found today.
Value of a natural pearl
Quality natural pearls are very rare jewels. The actual value of a natural pearl is determined in the same way as it would be for other "precious" gems. The valuation factors include size, shape, quality of surface, orient and luster.
Single natural pearls are often sold as a collector's item, or set as centerpieces in unique jewelry. Very few matched strands of natural pearls exist, and those that do often sell for hundreds of thousands of dollars. Yachtsman and financier Cartier purchased the landmark Cartier store on Fifth Avenue in New York for $100 cash and a double strand of matched natural pearls valued at $1 million.
Keshi pearls, although they often occur by chance, are not considered natural pearls. They are a byproduct of the culturing process, and hence do not happen without human intervention. These pearls are quite small: typically a few millimeters in size. Keshi pearls are produced by many different types of marine mollusks and freshwater mussels in China.[4] Today many "keshi" pearls are actually intentional, with post-harvest shells returned to the water to regenerate a pearl in the existing pearl sac.
Single natural pearls are often sold as a collector's item, or set as centerpieces in unique jewelry. Very few matched strands of natural pearls exist, and those that do often sell for hundreds of thousands of dollars. Yachtsman and financier Cartier purchased the landmark Cartier store on Fifth Avenue in New York for $100 cash and a double strand of matched natural pearls valued at $1 million.
Keshi pearls, although they often occur by chance, are not considered natural pearls. They are a byproduct of the culturing process, and hence do not happen without human intervention. These pearls are quite small: typically a few millimeters in size. Keshi pearls are produced by many different types of marine mollusks and freshwater mussels in China.[4] Today many "keshi" pearls are actually intentional, with post-harvest shells returned to the water to regenerate a pearl in the existing pearl sac.
Gemological identification
A well equipped gem testing laboratory (e.g. SSEF, Guebelin, GIA, AGTA, HIRCO-INDIA) is able to distinguish natural pearls from cultured pearls by using a gemological x-ray in order to examine the center of a pearl. With an x-ray it is possible to see the growth rings of the pearl, where the layers of calcium carbonate are separated by thin layers of conchiolin. The differentiation of natural pearls from tissue-nucleated cultured pearls can be very difficult without the use of this x-ray technique.
Natural and cultured pearls can be distinguished from imitation pearls using a microscope. Another method of testing for imitations is to rub the pearl against the surface of a front tooth. Imitation pearls are completely smooth, but natural and cultured pearls are composed of nacre platelets, which feel slightly gritty.
Natural and cultured pearls can be distinguished from imitation pearls using a microscope. Another method of testing for imitations is to rub the pearl against the surface of a front tooth. Imitation pearls are completely smooth, but natural and cultured pearls are composed of nacre platelets, which feel slightly gritty.
Cultured pearls
Cultured pearls (nucleated and non-nucleated or tissue nucleated cultured pearls) and imitation pearls can be distinguished from natural pearls by X-ray examination. Nucleated cultured pearls are often 'pre-formed' as they tend to follow the shape of the implanted shell bead nucleus. Once the pre-formed beads are inserted into the oyster, it secretes a few layers of nacre around the outside surface of the implant before it is removed after six months or more.
When a nucleated cultured pearl is X-rayed, it reveals a different structure to that of a natural pearl. A cultured pearl shows a solid center with no concentric growth rings, whereas a natural pearl shows a series of concentric growth rings.
When a nucleated cultured pearl is X-rayed, it reveals a different structure to that of a natural pearl. A cultured pearl shows a solid center with no concentric growth rings, whereas a natural pearl shows a series of concentric growth rings.
Natural pearls
Natural pearls are nearly 100% calcium carbonate and conchiolin. It is thought that natural pearls form under a set of accidental conditions when a microscopic intruder or parasite enters a bivalve mollusk, and settles inside the shell. The mollusk, being irritated by the intruder, secretes the calcium carbonate and conchiolin to cover the irritant. This secretion process is repeated many times, thus producing a pearl. Natural pearls come in many shapes, with perfectly round ones being comparatively rare.
Creation of a pearl
The difference between natural and cultured pearls focuses on whether the pearl was created spontaneously by nature — without human intervention — or with human aid. Pearls are formed inside the shell of certain mollusks: as a defense mechanism to a potentially threatening irritant such as a parasite inside its shell, the mollusk creates a pearl to seal off the irritation.
The mantle of the mollusk deposits layers of calcium carbonate (CaCO3) in the form of the mineral aragonite or a mixture of aragonite and calcite (both crystalline forms of calcium carbonate) held together by an organic horn-like compound called conchiolin. The combination of aragonite and conchiolin is called nacre, which makes up mother-of-pearl. The commonly held belief that a grain of sand acts as the irritant is in fact rarely the case. Typical stimuli include organic material, parasites, or even damage that displaces mantle tissue to another part of the animal's body. These small particles or organisms enter the animal when the shell valves are open for feeding or respiration. In cultured pearls, the irritant is typically a cut piece of the mantle epithelium, together with processed shell beads, the combination of which the animal accepts into its body.
The mantle of the mollusk deposits layers of calcium carbonate (CaCO3) in the form of the mineral aragonite or a mixture of aragonite and calcite (both crystalline forms of calcium carbonate) held together by an organic horn-like compound called conchiolin. The combination of aragonite and conchiolin is called nacre, which makes up mother-of-pearl. The commonly held belief that a grain of sand acts as the irritant is in fact rarely the case. Typical stimuli include organic material, parasites, or even damage that displaces mantle tissue to another part of the animal's body. These small particles or organisms enter the animal when the shell valves are open for feeding or respiration. In cultured pearls, the irritant is typically a cut piece of the mantle epithelium, together with processed shell beads, the combination of which the animal accepts into its body.
Freshwater and saltwater pearls
Freshwater and saltwater pearls may sometimes look quite similar, but they come from very different sources.
Natural freshwater pearls form in various species of freshwater mussels, family Unionidae, which live in lakes, rivers, ponds and other bodies of fresh water. These freshwater pearl mussels occur not only in hotter climates, but also in colder more temperate areas such as Scotland: see the freshwater pearl mussel. However, most freshwater cultured pearls sold today come from China.
Saltwater pearls grow within pearl oysters, family Pteriidae, which live in oceans. Saltwater pearl oysters are usually cultivated in protected lagoons or volcanic atolls.
Natural freshwater pearls form in various species of freshwater mussels, family Unionidae, which live in lakes, rivers, ponds and other bodies of fresh water. These freshwater pearl mussels occur not only in hotter climates, but also in colder more temperate areas such as Scotland: see the freshwater pearl mussel. However, most freshwater cultured pearls sold today come from China.
Saltwater pearls grow within pearl oysters, family Pteriidae, which live in oceans. Saltwater pearl oysters are usually cultivated in protected lagoons or volcanic atolls.
Physical properties
The unique luster of pearls depends upon the reflection, refraction, and diffraction of light from the translucent layers. The thinner and more numerous the layers in the pearl, the finer the luster. The iridescence that pearls display is caused by the overlapping of successive layers, which breaks up light falling on the surface.
In addition, pearls (especially cultured freshwater pearls) can be dyed yellow, green, blue, brown, pink, purple, or black.
In addition, pearls (especially cultured freshwater pearls) can be dyed yellow, green, blue, brown, pink, purple, or black.
Definition of a pearl
Almost any shelled mollusk can, by natural processes, produce some kind of "pearl" when an irritating microscopic object becomes trapped within the mollusk's mantle folds, but the great majority of these "pearls" are not valued as gemstones. Nacreous pearls, the best-known and most commercially-significant pearls, are primarily produced by two groups of molluscan bivalves or clams. A nacreous pearl is made from layers of nacre, by the same living process as is used in the secretion of the mother of pearl which lines the shell.
A "natural pearl" is one that forms without any human intervention at all, in the wild, and is very rare. Many hundreds of pearl oysters or pearl mussels have to be gathered and opened, and thus killed, in order to find even one wild pearl, and for many centuries that was the only way pearls were obtained. This was the main reason why pearls fetched such extraordinary prices in the past. A cultured pearl, on the other hand, is one that has been formed on a pearl farm. In modern times however, almost all the pearls for sale were formed with the aid of human pearl farmers. The great majority of pearls on the market are cultured pearls.
One family of nacreous pearl bivalves, the pearl oysters, lives in the sea while the other, very different group of bivalves live in freshwater; these are the river mussels such as the freshwater pearl mussel. Saltwater pearls can grow in several species of marine pearl oysters in the family Pteriidae. Freshwater pearls grow within certain (but by no means all) species of freshwater mussels in the order Unionida, the families Unionidae and Margaritiferidae.
A "natural pearl" is one that forms without any human intervention at all, in the wild, and is very rare. Many hundreds of pearl oysters or pearl mussels have to be gathered and opened, and thus killed, in order to find even one wild pearl, and for many centuries that was the only way pearls were obtained. This was the main reason why pearls fetched such extraordinary prices in the past. A cultured pearl, on the other hand, is one that has been formed on a pearl farm. In modern times however, almost all the pearls for sale were formed with the aid of human pearl farmers. The great majority of pearls on the market are cultured pearls.
One family of nacreous pearl bivalves, the pearl oysters, lives in the sea while the other, very different group of bivalves live in freshwater; these are the river mussels such as the freshwater pearl mussel. Saltwater pearls can grow in several species of marine pearl oysters in the family Pteriidae. Freshwater pearls grow within certain (but by no means all) species of freshwater mussels in the order Unionida, the families Unionidae and Margaritiferidae.
Pearl
A pearl is a hard, roundish object produced within the soft tissue (specifically the mantle) of a living shelled mollusk. Just like the shell of mollusks, a pearl is made up of of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes of pearls (baroque pearls) occur. The finest quality natural pearls have been highly valued as gemstones and objects of beauty for many centuries, and because of this, the word pearl became a metaphor for something very rare, very fine, very admirable and very valuable.
Valuable pearls occur in the wild, but they are very rare. Cultured or farmed pearls make up the majority of those that are currently sold. Pearls from the sea are valued more highly than freshwater pearls. Imitation or fake pearls are also widely sold in inexpensive jewelry, but the quality of the iridescence is usually very poor, and generally speaking, fake pearls are usually quite easy to distinguish from the real thing. Pearls have been harvested, or more recently cultivated, primarily for use in jewelry, but in the past they were also stitched onto lavish clothing, as worn, for example, by royalty. Pearls have also been crushed and used in cosmetics, medicines, or in paint formulations.
In several European languages, the word "pearl" is synonymous with "bead", which can lead to confusion during translation.
Valuable pearls occur in the wild, but they are very rare. Cultured or farmed pearls make up the majority of those that are currently sold. Pearls from the sea are valued more highly than freshwater pearls. Imitation or fake pearls are also widely sold in inexpensive jewelry, but the quality of the iridescence is usually very poor, and generally speaking, fake pearls are usually quite easy to distinguish from the real thing. Pearls have been harvested, or more recently cultivated, primarily for use in jewelry, but in the past they were also stitched onto lavish clothing, as worn, for example, by royalty. Pearls have also been crushed and used in cosmetics, medicines, or in paint formulations.
In several European languages, the word "pearl" is synonymous with "bead", which can lead to confusion during translation.
Famous opals
The Andamooka Opal, presented to Queen Elizabeth II, also known as the Queen's Opal
The Aurora Australis Opal, considered to be the most valuable black opal
The Black Prince Opal, originally known as Harlequin Prince
The Empress of Australia Opal
The Fire Queen Opal
The Flame Queen Opal
The Flamingo Opal
The Halley's Comet Opal, the world's largest uncut black opal
The Jupiter Five Opal
The Olympic Australis Opal, reported to be the largest and most valuable uncut gem opal ever found
The Pride of Australia Opal, also known as the Red Emperor Opal
The Red Admiral Opal, also known as the Butterfly Stone
The Roebling Opal, Smithsonian Museum [12]
The Aurora Australis Opal, considered to be the most valuable black opal
The Black Prince Opal, originally known as Harlequin Prince
The Empress of Australia Opal
The Fire Queen Opal
The Flame Queen Opal
The Flamingo Opal
The Halley's Comet Opal, the world's largest uncut black opal
The Jupiter Five Opal
The Olympic Australis Opal, reported to be the largest and most valuable uncut gem opal ever found
The Pride of Australia Opal, also known as the Red Emperor Opal
The Red Admiral Opal, also known as the Butterfly Stone
The Roebling Opal, Smithsonian Museum [12]
Historical superstitions
In the Middle Ages, opal was considered a stone that could provide great luck because it was believed to possess all the virtues of each gemstone whose color was represented in the color spectrum of the opal.[11] Victorian superstitions were created by the established gem dealers to stop the rush to buy opals. They paid an author to attribute bad luck to the stone, though some believed this is avoided if opal is the owner's birthstone or if the stone was a gift. Even as recently as under the last czar at the beginning of the 20th century, it was believed that when a Russian of any rank saw an opal among other goods offered for sale, he or she should not buy anything more since the opal was believed to embody the evil eye.[11] Opal is considered the birthstone for people born in October.
Non-crystalline opal
Two broad categories of non-crystalline opals, sometimes just referred to as "opal-A", have been proposed. The first of these is opal-AG consisting of aggregated spheres of silica, with water filling the space in between. Precious opal and potch opal are generally varieties of this, the difference being in the regularity of the sizes of the spheres and their packing. The second "opal-A" is opal-AN or water-containing amorphous silica-glass. Hyalite is another name for this.
Non-crystalline silica in siliceous sediments is reported to gradually transform to opal-CT and then opal-C as a result of diagenesis, due to the increasing overburden pressure in sedimentary rocks, as some of the stacking disorder is removed.[10]
Microcrystalline opal
Opal-CT has been interpreted as consisting of clusters of stacking of cristobalite and tridymite over very short length scales. The spheres of opal in opal-CT are themselves made up of tiny microcrystalline blades of cristobalite and tridymite. Opal-CT has occasionally been further subdivided in the literature. Water content may be as high as 10 wt%. Lussatite is a synonym. Opal-C, also called Lussatine, is interpreted as consisting of localized order of α-cristobalite with a lot of stacking disorder. Typical water content is about 1.5wt%.
Local atomic structure of opals
The lattice of spheres of opal that cause the interference with light are several hundred times larger than the fundamental structure of crystalline silica. As a mineraloid, there is no unit cell that describes the structure of opal. Nevertheless, opals can be roughly divided into those that show no signs of crystalline order (amorphous opal) and those that show signs of the beginning of crystalline order, commonly termed cryptocrystalline or microcrystalline opal.[9] Dehydration experiments and infrared spectroscopy have shown that most of the H2O in the formula of SiO2·nH2O of opals is present in the familiar form of clusters of molecular water. Isolated water molecules, and silanols, structures such as Si-O-H, generally form a lesser proportion of the total and can reside near the surface or in defects inside the opal.
The structure of low-pressure polymorphs of anhydrous silica consist of frameworks of fully-corner bonded tetrahedra of SiO4. The higher temperature polymorphs of silica cristobalite and tridymite are frequently the first to crystallize from amorphous anhydrous silica, and the local structures of microcrystalline opals also appear to be closer to that of cristobalite and tridymite than to quartz. The structures of tridymite and cristobalite are closely related and can be described as hexagonal and cubic close-packed layers. It is therefore possible to have intermediate structures in which the layers are not regularly stacked.
The structure of low-pressure polymorphs of anhydrous silica consist of frameworks of fully-corner bonded tetrahedra of SiO4. The higher temperature polymorphs of silica cristobalite and tridymite are frequently the first to crystallize from amorphous anhydrous silica, and the local structures of microcrystalline opals also appear to be closer to that of cristobalite and tridymite than to quartz. The structures of tridymite and cristobalite are closely related and can be described as hexagonal and cubic close-packed layers. It is therefore possible to have intermediate structures in which the layers are not regularly stacked.
Synthetic opal
As well as occurring naturally, opals of all varieties have been synthesized experimentally and commercially. The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974.[5] The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of color are seen to be arranged in a "lizard skin" or "chicken wire" pattern. Synthetics are further distinguished from naturals by the former's lack of fluorescence under UV light. Synthetics are also generally lower in density and are often highly porous.
Two notable producers of synthetic opal are the companies Kyocera and Inamori of Japan. Most so-called synthetics, however, are more correctly termed "imitation opal", as they contain substances not found in natural opal (e.g., plastic stabilizers). The imitation opals seen in vintage jewelry are often "Slocum Stone" consisting of laminated glass with bits of foil interspersed.
Two notable producers of synthetic opal are the companies Kyocera and Inamori of Japan. Most so-called synthetics, however, are more correctly termed "imitation opal", as they contain substances not found in natural opal (e.g., plastic stabilizers). The imitation opals seen in vintage jewelry are often "Slocum Stone" consisting of laminated glass with bits of foil interspersed.
Sources of opal
Australia produces around 97% of the world’s opal. 90% is called ‘light opal’ or white and crystal opal. White makes up 60% of the opal productions but cannot be found in all of the opal fields. Crystal opal or pure hydrated silica makes up 30% of the opal produced, 8% is black and only 2% is boulder opal.[citation needed]
The town of Coober Pedy in South Australia is a major source of opal. Andamooka in South Australia is also a major producer of matrix opal, crystal opal, and black opal. Another Australian town, Lightning Ridge in New South Wales, is the main source of black opal, opal containing a predominantly dark background (dark-gray to blue-black displaying the play of color). Boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix. It is found sporadically in western Queensland, from Kynuna in the north, to Yowah and Koroit in the south.[7]
Multi-colored rough opal specimen from Virgin Valley, Nevada, USA
The Virgin Valley opal fields of Humboldt County in northern Nevada produce a wide variety of precious black, crystal, white, fire, and lemon opal. The black fire opal is the official gemstone of Nevada. Most of the precious opal is partial wood replacement. Miocene age opalised teeth, bones, fish, and a snake head have been found. Some of the opal has high water content and may desiccate and crack when dried. The largest black opal in the Smithsonian Museum comes from the Royal Peacock opal mine in the Virgin Valley.[citation needed]
Another source of white base opal in the United States is Spencer, Idaho. A high percentage of the opal found there occurs in thin layers. As a result, most of the production goes into the making of doublets and triplets.
Other significant deposits of precious opal around the world can be found in the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil, Honduras, Guatemala, Nicaragua and Ethiopia.
In late 2008, NASA announced that it had discovered opal deposits on Mars.[8]
The town of Coober Pedy in South Australia is a major source of opal. Andamooka in South Australia is also a major producer of matrix opal, crystal opal, and black opal. Another Australian town, Lightning Ridge in New South Wales, is the main source of black opal, opal containing a predominantly dark background (dark-gray to blue-black displaying the play of color). Boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix. It is found sporadically in western Queensland, from Kynuna in the north, to Yowah and Koroit in the south.[7]
Multi-colored rough opal specimen from Virgin Valley, Nevada, USA
The Virgin Valley opal fields of Humboldt County in northern Nevada produce a wide variety of precious black, crystal, white, fire, and lemon opal. The black fire opal is the official gemstone of Nevada. Most of the precious opal is partial wood replacement. Miocene age opalised teeth, bones, fish, and a snake head have been found. Some of the opal has high water content and may desiccate and crack when dried. The largest black opal in the Smithsonian Museum comes from the Royal Peacock opal mine in the Virgin Valley.[citation needed]
Another source of white base opal in the United States is Spencer, Idaho. A high percentage of the opal found there occurs in thin layers. As a result, most of the production goes into the making of doublets and triplets.
Other significant deposits of precious opal around the world can be found in the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil, Honduras, Guatemala, Nicaragua and Ethiopia.
In late 2008, NASA announced that it had discovered opal deposits on Mars.[8]
Other varieties of opal
Fire opals are transparent to translucent opals with warm body colors yellow, orange, orange-yellow or red and they do not show any play-of-color. The most famous source of fire opals is the state of Queretaro in Mexico and these opals are commonly called Mexican fire opals.
Peruvian opal (also called blue opal) is a semi-opaque to opaque blue-green stone found in Peru which is often cut to include the matrix in the more opaque stones. It does not display pleochroism.
Peruvian opal (also called blue opal) is a semi-opaque to opaque blue-green stone found in Peru which is often cut to include the matrix in the more opaque stones. It does not display pleochroism.
Common opal
Besides the gemstone varieties that show a play of color, there are other kinds of common opal such as the milk opal, milky bluish to greenish (which can sometimes be of gemstone quality), resin opal which is honey-yellow with a resinous luster, wood opal which is caused by the replacement of the organic material in wood with opal[6], menilite which is brown or grey, hyalite is a colorless glass-clear opal sometimes called Muller's Glass, geyserite, also called siliceous sinter, deposited around hot springs or geysers and diatomite or diatomaceous earth, the accumulations of diatom shells or tests.
Precious opal
Precious opal shows a variable interplay of internal colors and even though it is a mineraloid, it does have an internal structure. At the micro scale precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close-packed lattice. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal.[5] It is the regularity of the sizes and the packing of these spheres that determines the quality of precious opal. Where the distance between the regularly packed planes of spheres is approximately half the wavelength of a component of visible light, the light of that wavelength may be subject to diffraction from the grating created by the stacked planes. The spacing between the planes and the orientation of planes with respect to the incident light determines the colors observed. The process can be described by Bragg's Law of diffraction.
Precious opal consists of spheres of silica of fairly regular size, packed into close-packed planes which are stacked together with characteristic dimensions of several hundred nm.
Visible light of diffracted wavelengths cannot pass through large thicknesses of the opal. This is the basis of the optical band gap in a photonic crystal, of which opal is the best known natural example. In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during solidification. The term opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed play of color. Contrarily, opalescence is correctly applied to the milky, turbid appearance of common or potch opal. Potch does not show a play of color.
The veins of opal displaying the play of color are often quite thin, and this has given rise to unusual methods of preparing the stone as a gem. An opal doublet is a thin layer of opal, backed by a swart mineral such as ironstone, basalt, or obsidian. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch.
Combined with modern techniques of polishing, doublet opal produces similar effect of black or boulder opals at a mere fraction of the price. Doublet opal also has the added benefit of having genuine opal as the top visible and touchable layer, unlike triplet opals.
The triplet-cut opal backs the colored material with a dark backing, and then has a domed cap of clear quartz or plastic on top, which takes a high polish and acts as a protective layer for the relatively fragile opal. The top layer also acts as a magniflier, to emphasis the play of color of the opal beneath, which are often of a lower quality. Triplet opals therefore have a more artificial feel to it and are not classed as precious opa
Opal
Opal is a mineraloid gel which is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, and basalt. The word opal comes from the Latin opalus, by Greek opallios, and is from the same root as Sanskrit upálá[s] for "stone", originally a millstone with upárá[s] for slab.[4]
The water content is usually between three and ten percent, but can be as high as twenty percent. Opal ranges from clear through white, gray, red, orange, yellow, green, shore, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare, whereas white and greens are the most common. These color variations are a function of growth size into the red and infrared wavelengths. Common opal is truly amorphous, but precious opal does have a structural element. (see Upal). Opals are also Australia's national gemstone. Opal is one of the mineraloids that can form or replace fossils. The resulting fossils appeal to collectors, although remain less significant for many scientists.
The water content is usually between three and ten percent, but can be as high as twenty percent. Opal ranges from clear through white, gray, red, orange, yellow, green, shore, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare, whereas white and greens are the most common. These color variations are a function of growth size into the red and infrared wavelengths. Common opal is truly amorphous, but precious opal does have a structural element. (see Upal). Opals are also Australia's national gemstone. Opal is one of the mineraloids that can form or replace fossils. The resulting fossils appeal to collectors, although remain less significant for many scientists.
Emerald in different cultures, and emerald lore
Emerald is regarded as the traditional birthstone for May, as well as the traditional gemstone for the astrological signs of Taurus, Cancer and sometimes Gemini. One of the more quaint anecdotes on emeralds was by the 16th-century historian Brantome, who referred to the many impressive emeralds the Spanish under Cortez had brought back to Europe from Latin America. On one of Cortez's most notable emeralds he had the text engraved Inter Natos Mulierum non sur-rexit mayor (Among them borne of woman there hath not arisen a greater Man. XI, 11) which referred to John the Baptist. Brantome considered engraving such a beautiful and simple product of nature sacrilegious and considered this act the cause for Cortez's loss of an extremely precious pearl (to which he dedicated a work A beautiful and incomparable pearl) and even for the death of King Charles IX who died soon after.[10]
In some cultures, the emerald is the traditional gift for the 55th wedding anniversary. It is also used as a 20th and 35th wedding anniversary stone.
The Authorized King James Version of the Bible, in Exodus 28:18 and 39:11, lists "emerald" as one of the precious stones in the breastplate of the high priest of the Jews; but modern consensus is that this is probably a mistranslation. (See Hoshen.)
Ireland is often referred to, especially in America, as the "Emerald Isle".
Synthetic emerald
Emerald is a rare and valuable gemstone and, as such, it has provided the incentive for developing synthetic emeralds. Both hydrothermal and flux-growth synthetics have been produced, and a method has been developed for producing an emerald overgrowth on colorless beryl. The first commercially successful emerald synthesis process was that of Carroll Chatham. Because Chatham's emeralds do not have any water and contain traces of vanadate, molybdenum and vanadium, a lithium vanadate flux process is probably involved. The other large producer of flux emeralds is Pierre Gilson Sr., which has been on the market since 1964. Gilson's emeralds are usually grown on natural colorless beryl seeds which become coated on both sides. Growth occurs at the rate of 1 mm per month, a typical seven-month growth run producing emerald crystals of 7 mm of thickness (Nassau, K. Gems Made By Man, 1980).
Hydrothermal synthetic emeralds have been attributed to IG Farben, Nacken, Tairus, and others, but the first satisfactory commercial product was that of Johann Lechleitner of Innsbruck, Austria, which appeared on the market in the 1960s. These stones were initially sold under the names "Emerita" and "Symeralds", and they were grown as a thin layer of emerald on top of natural colorless beryl stones. Although not much is known about the original process, it is assumed that Leichleitner emeralds were grown in acid conditions. Later, from 1965 to 1970, the Linde Division of Union Carbide produced completely synthetic emeralds by hydrothermal synthesis. According to their patents (US3,567,642 and US3,567,643), acidic conditions are essential to prevent the chromium (which is used as the colorant) from precipitating. Also, it is important that the silicon-containing nutrient be kept away from the other ingredients to prevent nucleation and confine growth to the seed crystals. Growth occurs by a diffusion-reaction process, assisted by convection. Typical growth conditions include pressures of 700–1400 bars at temperatures of 500–600 °C with a temperature gradient of 10–25 °C. Growth rates as fast as 1/3 mm per day can be attained[citation needed]
Luminescence in ultraviolet light is considered a supplementary test when making a natural vs. synthetic determination, as many, but not all, natural emeralds are inert to ultraviolet light. Many synthetics are also UV inert.[8]
Synthetic emeralds are often referred to as "created", as their chemical and gemological composition is the same as their natural counterparts. The U.S. Federal Trade Commission (FTC) has very strict regulations as to what can and what cannot be called "synthetic" stone. The FTC says: "§ 23.23(c) It is unfair or deceptive to use the word "laboratory-grown," "laboratory-created," "[manufacturer name]-created," or "synthetic" with the name of any natural stone to describe any industry product unless such industry product has essentially the same optical, physical, and chemical properties as the stone named."[9]
Wispy veil-like inclusions are common in flux-grown synthetic emeralds.
Emerald localities
Emeralds in antiquity were mined by the Egyptians and in Austria, as well as Swat in northern Pakistan.[6][7]
A rare type of emerald known as a trapiche emerald is occasionally found in the mines of Colombia. A trapiche emerald exhibits a "star" pattern; it has raylike spokes of dark carbon impurities that give the emerald a six-pointed radial pattern. It is named for the trapiche, a grinding wheel used to process sugarcane in the region. Colombian emeralds are generally the most prized due to their transparency and fire. Some of the most rare emeralds come from three main emerald mining areas in Colombia: Muzo, Coscuez, and Chivor. Fine emeralds are also found in other countries, such as Zambia, Brazil, Zimbabwe, Madagascar, Pakistan, India, Afghanistan and Russia. In the US, emeralds can be found in Hiddenite, North Carolina. In 1998, emeralds were discovered in the Yukon.
Treatments
Most emeralds are oiled as part of the post lapidary process, in order to improve their clarity. Cedar oil, having a similar refractive index, is often used in this generally accepted practice. Other liquids, including synthetic oils and polymers with refractive indexes close to that of emerald such as Opticon are also used. The U.S. Federal Trade Commission requires the disclosure of this treatment when a treated emerald is sold.[5] The use of oil is traditional and largely accepted by the gem trade. Other treatments, for example the use of green-tinted oil, are not acceptable in the trade. The laboratory community has recently standardized the language for grading the clarity of emeralds. Gems are graded on a four step scale; none, minor, moderate and highly enhanced. Note that these categories reflect levels of enhancement not clarity. A gem graded none on the enhancement scale may still exhibit visible inclusions. Laboratories tend to apply these criteria differently. Some gem labs consider the mere presence of oil or polymers to constitute enhancement. Others may ignore traces of oil if the presence of the material does not materially improve the look of the gemstone.
Given that the vast majority of all emeralds are treated as described above, and the fact that two stones that appear to be similar in quality may actually be quite far apart in treatment level, a consumer considering a purchase of an expensive emerald is well advised to insist upon a treatment report from a reputable gemological laboratory. All other factors being equal, a high quality emerald with an enhancement level graded moderate should cost 40-50% less than an identical stone graded none.
Clarity
Emerald tends to have numerous inclusions and surface breaking fissures. Unlike diamond, where the loupe standard, i.e. 10X magnification is used to grade clarity, emerald is graded by eye. Thus, if an emerald has no visible inclusions to the eye (assuming 20-20 vision) it is considered flawless. Stones that lack surface breaking fissures are extremely rare and therefore almost all emeralds are treated, "oiled", to enhance the apparent clarity. Eye-clean stones of a vivid primary green hue (as described above) with no more than 15% of any secondary hue or combination (either blue or yellow) of a medium-dark tone command the highest prices.[4] This relative crystal non-uniformity makes emeralds more likely than other gemstones to be cut into cabochons, rather than faceted shapes.
Color
Scientifically speaking, color is divided into three components: hue, saturation and tone. Yellow and blue, the hues found adjacent to green on the spectral color wheel, are the normal secondary hues found in emerald. Emeralds occur in hues ranging from yellowish green to bluish green. The primary hue must, of course, be green. Only gems that are medium to dark in tone are considered emerald. Light toned gems are known by the species name, green beryl. In addition, the hue must be bright (vivid). Gray is the normal saturation modifier or mask found in emerald. A grayish green hue is a dull green hue
Properties determining value
Emeralds, like all colored gemstones, are graded using four basic parameters, the four Cs of Connoisseurship; Color, Cut, Clarity and Crystal. The last C, crystal is simply used as a synonym that begins with C for transparency or what gemologists call diaphaneity. Prior to the 20th Century jewelers used the term water as in "a gem of the finest water"[3] to express the combination of two qualities, color and crystal. Normally, in the grading of colored gemstones, color is by far the most important criterion. However, in the grading of emerald, crystal is considered a close second. Both are necessary conditions. A fine emerald must possess not only a pure verdant green hue as described below, but also a high degree of transparency to be considered a top gem.[4]
Emerald
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Emeralds are a variety of the mineral beryl (Be3Al2(SiO3)6,) colored green by trace amounts of chromium and sometimes vanadium.[1] Beryl has a hardness of 7.5 - 8 on the 10 point Mohs scale of mineral hardness.[1] Most emeralds are highly included, so their brittleness (resistance to breakage) is classified as generally poor. The word "emerald" comes from Latin smaragdus, via Greek smaragdos, its original source being a Semitic word izmargad or the Sanskrit word, marakata, meaning "emerald" or "green".[2]
List of diamonds
A number of large or extraordinary diamonds have gained fame, both as exquisite examples of the beautiful nature of diamonds, and because of the famous people who wore, bought, and sold them. These diamonds are often depicted in marketing materials aimed at the retail diamond customer. A partial list of famous diamonds in history follows.
Darya-ye Noor
Tiffany Yellow Diamond
The Allnatt Diamond, a large antique cushion-shaped brilliant Fancy Vivid Yellow diamond.
The Agra Diamond, antique cushion-shaped stellar brilliant, 28 carats.
The Amsterdam Diamond, a 33.74 carat (6.748 g) pear-shaped black diamond which sold for $352,000 in 2001.
The Archduke Joseph Diamond, antique cushion-shaped brilliant, 76 carats.
The Ashberg Diamond
The Aurora Butterfly of Peace
The Aurora Pyramid of Hope
The Beau Sancy, a 34-carat diamond not to be confused with the Sancy.
The Black Orlov, a 67.50 carat cushion-cut black diamond, also called the Eye of Brahma Diamond.
The Blue Heart Diamond, 30.82-carat heart brilliant. Part of the Smithsonian collection.
The Briolette of India Diamond, 90 carats.
The Centenary Diamond, modified heart-shaped brilliant, the world's largest colorless (grade D), flawless diamond.
The Chloe Diamond, largest round brilliant-cut diamond ever put on auction. Sold on November 14th, 2007 at Sotheby's in Geneva to Georges Marciano of the Guess clothing line for $16.2 million, the second-highest price ever paid for a diamond on auction. Took 2 years to cut.
The Cullinan Diamond, the largest rough gem-quality diamond ever found at 3106.75 carats (621.35 g). It was cut into 105 diamonds including the Cullinan I or the Great Star of Africa, 530.2 carats (106.04 g), and the Cullinan II or the Lesser Star of Africa, 317.4 carats (63.48 g), both of which are now part of the British Crown Jewels.
The Darya-ye Noor Diamond, the largest pink diamond in the world, about 186 carats (36.4 g), part of Iranian Crown Jewels. Its exact weight isn't known and 186 carats is an estimate.
The Deepdene, widely considered to be the largest artificially irradiated diamond in the world.
The De Young Red Diamond, the third-largest known red diamond was bought in a flea market on a hatpin by Sidney deYoung a prominent Boston estate jewelry merchant. It was donated by him to the museum of natural history.
The Dresden Green Diamond, antique pear-shaped brilliant - its color is the result of natural irradiation
The Dresden White Diamond, 47-carat antique oval brilliant, colorless/near-colorless
The Dresden Yellow Diamond, an antique round cut weighing 38 carats
The Earth Star Diamond a 111.59-carat, pear-shaped diamond with a strong coffee-like brown color.
The Empress Eugenie Diamond, 52-carat antique pear-shaped brilliant with an odd, random facet pattern
The Excelsior Diamond, the largest known diamond in the world prior to the Cullinan
The Florentine Diamond, a lost diamond, light yellow with a weight of 137.27 carats (27.45 g).
The Golden Jubilee Diamond, the largest faceted diamond ever cut at 545.67 carats (109.13 g)
The Graff Blue Diamond
The Great Chrysanthemum Diamond
The Great Mogul Diamond, fabled 280-carat mogul-cut diamond, now lost, although presumed by historians to have been recut as the Orlov.
The Gruosi Diamond, a heart-shaped black diamond, weighing 115.34 carats.
The Heart of Eternity Diamond, perhaps the largest Fancy Vivid Blue
The Hope Diamond, Fancy Dark Grayish-Blue and supposedly cursed. Almost certainly cut from the French Blue Diamond
The Hortensia Diamond, peach color, formerly part of the French Crown Jewels. Displayed in the Louvre.
The Idol's Eye
The Incomparable Diamond, a brownish-yellow diamond of 407.48 carats (81.496 g) cut from an 890 carat (178 g) rough diamond of the same name - it appeared on eBay in 2002. Internally Flawless clarity.
The Jacob Diamond weighing 184.5 carats (36.90 g), also known as Imperial Diamond & Victoria Diamond.
The Jones Diamond
The Jubilee Diamond, originally known as the Reitz Diamond; perhaps the sixth-largest in the world.
The Kimberley Diamond
The Koh-i-Noor, a 105 carat (21.6 g) white of Indian origin, with a long and turbulent history and a good deal of legend surrounding it. After belonging to various Mughal and Persian rulers, it was taken away from the Maharaja Duleep Singh of Lahore and was presented to Queen Victoria during the British Raj, and is now part of the Crown of Queen Elizabeth the Queen Mother.
The Lesotho
The Lesotho Promise, is the 15th-largest diamond, the tenth-largest white diamond, and the largest diamond to be found in 13 years
The Millennium Star, at 203 carats is the second-largest colorless (grade D), flawless diamond.
File:Pumpkindiamond4.jpg
Pumpkin Diamond
Koh-i-Noor (glass replica)
The Moon of Baroda
The Moussaieff Red Diamond, the largest known Fancy Red
The Mouna Diamond, 112 carats, Fancy Intense Yellow cushion-shaped brilliant.
The Nassak Diamond[1]
The Nepal Diamond, fine quality antique pear-shaped brilliant, sold by Harry Winston to private collector in 1961. Thought to have originated from the Golconda Mines.
The Nizam Diamond
The Nur-Ul-Ain Diamond
The Ocean Dream Diamond, the only known natural Fancy Deep Blue-Green.
The Oppenheimer Diamond, one of the largest gem-quality uncut diamonds in the world.
The Orlov, an Indian mogul cut rumored to have served as the eye of a Hindu statue.
The Paragon Diamond
The Porter Rhodes Diamond, a colorless 53-carat Asscher-cut stone.
The Portuguese Diamond, 127-carat antique emerald cut with a pale yellow body color and very strong blue fluorescence. Part of the Smithsonian's collection.
The Premier Rose Diamond, 137.02-carat (27.4 g) stone cut from a 353.9-carat (70.8 g) rough gem of the same name
The Pumpkin Diamond, perhaps the largest Fancy Vivid Orange. 5.54 carats, modified cushion-shaped brilliant.
The Red Cross Diamond, 205 carats, yellow, cushion-shaped stellar brilliant cut.
The Regent Diamond, cushion-shaped stellar brilliant cut, formerly belonging to Louis XV, Louis XVI, and Napoleon Bonaparte, it now resides in the Louvre.
The Sancy, a shield-shaped pale yellow diamond currently in the Louvre.
The Shah Diamond, very old yellow diamond (found approximately in 1450 in India) currently housed in the Diamond Fund in Kremlin
The Spirit of de Grisogono Diamond, 312 carats, the world's largest cut black diamond.
The Spoonmaker's Diamond, circa 86-carat (17 g) diamond housed in Topkapı Palace in Istanbul.
The Star of Arkansas
The Star of the East, a 95-carat (19 g) stone once owned by Mrs. Evalyn McLean of Washington DC, who also owned the Hope Diamond.
The Star of South Africa, also known as the Dudley Diamond. This must not be confused with the Star of Africa. The Star of South Africa was the initial name given to this diamond, when it was purchased as an 83.5-carat rough diamond. The diamond is a D-color, pear-shaped stellar brilliant cut stone, weighing 47.69 carats.
The Star of the South
The Steinmetz Pink Diamond, modified oval brilliant cut (step cut crown, brilliant pavilion), largest known Fancy Vivid Pink.
The Taylor-Burton Diamond
The Tereschenko, 42-carat antique pear brilliant cut.
The Tiffany Yellow Diamond, antique modified cushion-shaped stellar brilliant cut, on display at Tiffany & Co.'s New York City store.
The Uncle Sam Diamond, the largest discovered in the US, emerald-cut, M color (pale brown), VVS2 clarity.
The Vargas
The Wittelsbach Diamond, 35.52 carats, Fancy Deep Grayish Blue, antique oval stellar brilliant cut. Sold at Christie's, London, December 10, 2008 for $23.4 million, currently the highest price ever paid for a diamond at auction.
Darya-ye Noor
Tiffany Yellow Diamond
The Allnatt Diamond, a large antique cushion-shaped brilliant Fancy Vivid Yellow diamond.
The Agra Diamond, antique cushion-shaped stellar brilliant, 28 carats.
The Amsterdam Diamond, a 33.74 carat (6.748 g) pear-shaped black diamond which sold for $352,000 in 2001.
The Archduke Joseph Diamond, antique cushion-shaped brilliant, 76 carats.
The Ashberg Diamond
The Aurora Butterfly of Peace
The Aurora Pyramid of Hope
The Beau Sancy, a 34-carat diamond not to be confused with the Sancy.
The Black Orlov, a 67.50 carat cushion-cut black diamond, also called the Eye of Brahma Diamond.
The Blue Heart Diamond, 30.82-carat heart brilliant. Part of the Smithsonian collection.
The Briolette of India Diamond, 90 carats.
The Centenary Diamond, modified heart-shaped brilliant, the world's largest colorless (grade D), flawless diamond.
The Chloe Diamond, largest round brilliant-cut diamond ever put on auction. Sold on November 14th, 2007 at Sotheby's in Geneva to Georges Marciano of the Guess clothing line for $16.2 million, the second-highest price ever paid for a diamond on auction. Took 2 years to cut.
The Cullinan Diamond, the largest rough gem-quality diamond ever found at 3106.75 carats (621.35 g). It was cut into 105 diamonds including the Cullinan I or the Great Star of Africa, 530.2 carats (106.04 g), and the Cullinan II or the Lesser Star of Africa, 317.4 carats (63.48 g), both of which are now part of the British Crown Jewels.
The Darya-ye Noor Diamond, the largest pink diamond in the world, about 186 carats (36.4 g), part of Iranian Crown Jewels. Its exact weight isn't known and 186 carats is an estimate.
The Deepdene, widely considered to be the largest artificially irradiated diamond in the world.
The De Young Red Diamond, the third-largest known red diamond was bought in a flea market on a hatpin by Sidney deYoung a prominent Boston estate jewelry merchant. It was donated by him to the museum of natural history.
The Dresden Green Diamond, antique pear-shaped brilliant - its color is the result of natural irradiation
The Dresden White Diamond, 47-carat antique oval brilliant, colorless/near-colorless
The Dresden Yellow Diamond, an antique round cut weighing 38 carats
The Earth Star Diamond a 111.59-carat, pear-shaped diamond with a strong coffee-like brown color.
The Empress Eugenie Diamond, 52-carat antique pear-shaped brilliant with an odd, random facet pattern
The Excelsior Diamond, the largest known diamond in the world prior to the Cullinan
The Florentine Diamond, a lost diamond, light yellow with a weight of 137.27 carats (27.45 g).
The Golden Jubilee Diamond, the largest faceted diamond ever cut at 545.67 carats (109.13 g)
The Graff Blue Diamond
The Great Chrysanthemum Diamond
The Great Mogul Diamond, fabled 280-carat mogul-cut diamond, now lost, although presumed by historians to have been recut as the Orlov.
The Gruosi Diamond, a heart-shaped black diamond, weighing 115.34 carats.
The Heart of Eternity Diamond, perhaps the largest Fancy Vivid Blue
The Hope Diamond, Fancy Dark Grayish-Blue and supposedly cursed. Almost certainly cut from the French Blue Diamond
The Hortensia Diamond, peach color, formerly part of the French Crown Jewels. Displayed in the Louvre.
The Idol's Eye
The Incomparable Diamond, a brownish-yellow diamond of 407.48 carats (81.496 g) cut from an 890 carat (178 g) rough diamond of the same name - it appeared on eBay in 2002. Internally Flawless clarity.
The Jacob Diamond weighing 184.5 carats (36.90 g), also known as Imperial Diamond & Victoria Diamond.
The Jones Diamond
The Jubilee Diamond, originally known as the Reitz Diamond; perhaps the sixth-largest in the world.
The Kimberley Diamond
The Koh-i-Noor, a 105 carat (21.6 g) white of Indian origin, with a long and turbulent history and a good deal of legend surrounding it. After belonging to various Mughal and Persian rulers, it was taken away from the Maharaja Duleep Singh of Lahore and was presented to Queen Victoria during the British Raj, and is now part of the Crown of Queen Elizabeth the Queen Mother.
The Lesotho
The Lesotho Promise, is the 15th-largest diamond, the tenth-largest white diamond, and the largest diamond to be found in 13 years
The Millennium Star, at 203 carats is the second-largest colorless (grade D), flawless diamond.
File:Pumpkindiamond4.jpg
Pumpkin Diamond
Koh-i-Noor (glass replica)
The Moon of Baroda
The Moussaieff Red Diamond, the largest known Fancy Red
The Mouna Diamond, 112 carats, Fancy Intense Yellow cushion-shaped brilliant.
The Nassak Diamond[1]
The Nepal Diamond, fine quality antique pear-shaped brilliant, sold by Harry Winston to private collector in 1961. Thought to have originated from the Golconda Mines.
The Nizam Diamond
The Nur-Ul-Ain Diamond
The Ocean Dream Diamond, the only known natural Fancy Deep Blue-Green.
The Oppenheimer Diamond, one of the largest gem-quality uncut diamonds in the world.
The Orlov, an Indian mogul cut rumored to have served as the eye of a Hindu statue.
The Paragon Diamond
The Porter Rhodes Diamond, a colorless 53-carat Asscher-cut stone.
The Portuguese Diamond, 127-carat antique emerald cut with a pale yellow body color and very strong blue fluorescence. Part of the Smithsonian's collection.
The Premier Rose Diamond, 137.02-carat (27.4 g) stone cut from a 353.9-carat (70.8 g) rough gem of the same name
The Pumpkin Diamond, perhaps the largest Fancy Vivid Orange. 5.54 carats, modified cushion-shaped brilliant.
The Red Cross Diamond, 205 carats, yellow, cushion-shaped stellar brilliant cut.
The Regent Diamond, cushion-shaped stellar brilliant cut, formerly belonging to Louis XV, Louis XVI, and Napoleon Bonaparte, it now resides in the Louvre.
The Sancy, a shield-shaped pale yellow diamond currently in the Louvre.
The Shah Diamond, very old yellow diamond (found approximately in 1450 in India) currently housed in the Diamond Fund in Kremlin
The Spirit of de Grisogono Diamond, 312 carats, the world's largest cut black diamond.
The Spoonmaker's Diamond, circa 86-carat (17 g) diamond housed in Topkapı Palace in Istanbul.
The Star of Arkansas
The Star of the East, a 95-carat (19 g) stone once owned by Mrs. Evalyn McLean of Washington DC, who also owned the Hope Diamond.
The Star of South Africa, also known as the Dudley Diamond. This must not be confused with the Star of Africa. The Star of South Africa was the initial name given to this diamond, when it was purchased as an 83.5-carat rough diamond. The diamond is a D-color, pear-shaped stellar brilliant cut stone, weighing 47.69 carats.
The Star of the South
The Steinmetz Pink Diamond, modified oval brilliant cut (step cut crown, brilliant pavilion), largest known Fancy Vivid Pink.
The Taylor-Burton Diamond
The Tereschenko, 42-carat antique pear brilliant cut.
The Tiffany Yellow Diamond, antique modified cushion-shaped stellar brilliant cut, on display at Tiffany & Co.'s New York City store.
The Uncle Sam Diamond, the largest discovered in the US, emerald-cut, M color (pale brown), VVS2 clarity.
The Vargas
The Wittelsbach Diamond, 35.52 carats, Fancy Deep Grayish Blue, antique oval stellar brilliant cut. Sold at Christie's, London, December 10, 2008 for $23.4 million, currently the highest price ever paid for a diamond at auction.
Synthetic and artificial gemstones
Some gemstones are manufactured to imitate other gemstones. For example, cubic zirconia is a synthetic diamond simulant composed of zirconium oxide. Moissanite is another example. The imitations copy the look and color of the real stone but possess neither their chemical nor physical characteristics.
However, lab created gemstones are not imitations. For example, diamonds, ruby, sapphires and emeralds have been manufactured in labs to possess identical chemical and physical characteristics to the naturally occurring variety. Synthetic (lab created) corundums, including ruby and sapphire, are very common and they cost only a fraction of the natural stones. Smaller synthetic diamonds have been manufactured in large quantities as industrial abrasives. Larger synthetic diamonds of gemstone quality, especially of the colored variety, are also manufactured.[citation needed]
Whether a gemstone is a natural stone or a lab-created (synthetic) stone, the characteristics of each are the same. Lab-created stones tend to have a more vivid color to them, as impurities are not present in a lab, so therefore do not affect the clarity or color of the stone. However, natural gemstones are still considered more valuable on average due to their relative scarcity.[citation needed]
Fracture filling
Fracture filling has been in use with different gemstones such as diamonds, emeralds and sapphires. More recently (in 2006) "Glass Filled Rubies" received a lot of publicity. Rubies over 10 Carat (2 g), particularly sold in the Asian market with large fractures were filled with lead glass, thus dramatically improving the appearance (of larger rubies in particular). Such treatments are fairly easy to detect.
Waxing/oiling
Emeralds containing natural fissures are sometimes filled with wax or oil to disguise them. This wax or oil is also colored to make the emerald appear of better color as well as clarity. Turquoise is also commonly treated in a similar manner.
Radiation
Most blue topaz, both the lighter and the darker blue shades such as "London" blue, has been irradiated to change the color from white to blue. Some improperly handled gems which do not pass through normal legal channels may have a slight residual radiation[citation needed], though strong requirements on imported stones are in place to ensure public safety. Most greened quartz (Oro Verde) is also irradiated to achieve the yellow-green color.
Heat
Heat can improve gemstone color or clarity. Most citrine is made by heating amethyst, and partial heating with a strong gradient results in ametrine - a stone partly amethyst and partly citrine. Much aquamarine is heat treated to remove yellow tones, change the green color into the more desirable blue or enhance its existing blue color to a purer blue. [12]
Nearly all tanzanite is heated at low temperatures to remove brown undertones and give a more desirable blue/purple color. A considerable portion of all sapphire and ruby is treated with a variety of heat treatments to improve both color and clarity.
When jewelry containing diamonds is heated (for repairs) the diamond should be protected with boracic acid; otherwise the diamond (which is pure carbon) could be burned on the surface or even burned completely up. When jewelry containing sapphires or rubies is heated (for repairs) it should not be coated with boracic acid or any other substance, as this can etch the surface; it does not have to be "protected" like a diamond.
Treatments applied to gemstones
Gemstones are often treated to enhance the color or clarity of the stone. Depending on the type and extent of treatment, they can affect the value of the stone. Some treatments are used widely because the resulting gem is stable, while others are not accepted most commonly because the gem color is unstable and may revert to the original tone.[11]
Gemstone color
Color is the most obvious and attractive feature of gemstones. The color of any material is due to the nature of light itself. Daylight, often called white light, is actually a mixture of different colors of light. When light passes through a material, some of the light may be absorbed, while the rest passes through. The part that is not absorbed reaches the eye as white light minus the absorbed colors. A ruby appears red because it absorbs all the other colors of white light - blue, yellow, green, etc. - except red.
The same material can exhibit different colors. For example ruby and sapphire have the same chemical composition (both are corundum) but exhibit different colors. Even the same gemstone can occur in many different colors: sapphires show different shades of blue and pink and "fancy sapphires" exhibit a whole range of other colors from yellow to orange-pink, the latter called "Padparadscha sapphire".
This difference in color is based on the atomic structure of the stone. Although the different stones formally have the same chemical composition, they are not exactly the same. Every now and then an atom is replaced by a completely different atom (and this could be as few as one in a million atoms). These so called impurities are sufficient to absorb certain colors and leave the other colors unaffected.
As an example: beryl, which is colorless in its pure mineral form, becomes emerald with chromium impurities. If you add manganese instead of chromium, beryl becomes pink morganite. With iron, it becomes aquamarine.
Some gemstone treatments make use of the fact that these impurities can be "manipulated", thus changing the color of the gem.
The same material can exhibit different colors. For example ruby and sapphire have the same chemical composition (both are corundum) but exhibit different colors. Even the same gemstone can occur in many different colors: sapphires show different shades of blue and pink and "fancy sapphires" exhibit a whole range of other colors from yellow to orange-pink, the latter called "Padparadscha sapphire".
This difference in color is based on the atomic structure of the stone. Although the different stones formally have the same chemical composition, they are not exactly the same. Every now and then an atom is replaced by a completely different atom (and this could be as few as one in a million atoms). These so called impurities are sufficient to absorb certain colors and leave the other colors unaffected.
As an example: beryl, which is colorless in its pure mineral form, becomes emerald with chromium impurities. If you add manganese instead of chromium, beryl becomes pink morganite. With iron, it becomes aquamarine.
Some gemstone treatments make use of the fact that these impurities can be "manipulated", thus changing the color of the gem.
Cutting and polishing
A few gemstones are used as gems in the crystal or other form in which they are found. Most however, are cut and polished for usage as jewelry. The two main classifications are stones cut as smooth, dome shaped stones called cabochons, and stones which are cut with a faceting machine by polishing small flat windows called facets at regular intervals at exact angles.
Stones which are opaque such as opal, turquoise, variscite, etc. are commonly cut as cabochons. These gems are designed to show the stone's color or surface properties as in opal and star sapphires. Grinding wheels and polishing agents are used to grind, shape and polish the smooth dome shape of the stones.[9]
Gems which are transparent are normally faceted, a method which shows the optical properties of the stone’s interior to its best advantage by maximizing reflected light which is perceived by the viewer as sparkle. There are many commonly used shapes for faceted stones. The facets must be cut at the proper angles, which varies depending on the optical properties of the gem. If the angles are too steep or too shallow, the light will pass through and not be reflected back toward the viewer. Special equipment, a faceting machine, is used to hold the stone onto a flat lap for cutting and polishing the flat facets.[10] Rarely, some cutters use special curved laps to cut and polish curved facets.
Stones which are opaque such as opal, turquoise, variscite, etc. are commonly cut as cabochons. These gems are designed to show the stone's color or surface properties as in opal and star sapphires. Grinding wheels and polishing agents are used to grind, shape and polish the smooth dome shape of the stones.[9]
Gems which are transparent are normally faceted, a method which shows the optical properties of the stone’s interior to its best advantage by maximizing reflected light which is perceived by the viewer as sparkle. There are many commonly used shapes for faceted stones. The facets must be cut at the proper angles, which varies depending on the optical properties of the gem. If the angles are too steep or too shallow, the light will pass through and not be reflected back toward the viewer. Special equipment, a faceting machine, is used to hold the stone onto a flat lap for cutting and polishing the flat facets.[10] Rarely, some cutters use special curved laps to cut and polish curved facets.
Tuesday, March 31, 2009
Grading
In the last two decades there has been a proliferation of certification, not only for diamonds but for gemstones as well. There are a number of [6] reputable laboratories which grade and provide reports on diamonds. As there is no universally accepted grading system for colored gemstones, only one laboratory, AGL (see below) grades gemstones for quality using a proprietary system developed by the lab.
International Gemological Institute (IGI), world's largest independent laboratory for grading and evaluation of diamonds, jewellery and colored stones.
Gemological Institute of America (GIA), the main provider of education services and diamond grading reports
American Gemological Society (AGS) is not as widely recognized nor as old as the GIA but has a high reputation.
American Gem Trade Laboratory which is part of the American Gem Trade Association (AGTA) the largest trade organization of jewelers and dealers of colored stones
American Gemological Laboratories (AGL) which was recently taken over by "Collector's Universe" a NASDAQ listed company which specializes in certification of many collectables such as coins and stamps
European Gemological Laboratory (EGL).
Gemological Association of All Japan (GAAJ), aka Zenhokyo, the preferred lab within Japan, also very active in the gemological research
Gemmological Institute of Thailand (GIT) is closely related to Chulalongkorn University, and has a good reputation for their gemmological research
Asian Institute of Gemmological Sciences (AIGS), the oldest gemological institute in South East Asia, involved in gemological education and gem testing
Swiss Gemmological Institute (SSEF), founded by Prof. Henry Hänni, offering a high scientific standard, and focusing on coloured gemstones and the identification of natural pearls
Gübelin Gem Lab, the traditional Swiss lab founded by the famous Dr. Eduard Gübelin. Their reports are widely considered as the ultimate judgement on high-end pearls, coloured gemstones and diamonds
Each laboratory has its own methodology to evaluate gemstones. Consequently a stone can be called "pink" by one lab while another lab calls it "Padparadscha". One lab can conclude a stone is untreated, while another lab concludes that it is heat treated [6]. To minimise such differences, seven of the most respected labs, i.e. AGTA-GTL (New York), CISGEM (Milano), GAAJ (Tokyo), GIA (Carlsbad), GIT (Bangkok), Gübelin (Lucerne) and SSEF (Basel), have established the Laboratory Manual Harmonisation Committee (LMHC), aiming at the standardisation of wording on reports and certain analytical methods and interpretation of results. Country of origin has sometimes been difficult to find agreement on due to the constant discovery of new locations. Moreover determining a "country of origin" is much more difficult than determining other aspects of a gem (such as cut, clarity etc.) [8].
Gem dealers are aware of the differences between gem laboratories and will make use of the discrepancies to obtain the best possible certificate [6].
International Gemological Institute (IGI), world's largest independent laboratory for grading and evaluation of diamonds, jewellery and colored stones.
Gemological Institute of America (GIA), the main provider of education services and diamond grading reports
American Gemological Society (AGS) is not as widely recognized nor as old as the GIA but has a high reputation.
American Gem Trade Laboratory which is part of the American Gem Trade Association (AGTA) the largest trade organization of jewelers and dealers of colored stones
American Gemological Laboratories (AGL) which was recently taken over by "Collector's Universe" a NASDAQ listed company which specializes in certification of many collectables such as coins and stamps
European Gemological Laboratory (EGL).
Gemological Association of All Japan (GAAJ), aka Zenhokyo, the preferred lab within Japan, also very active in the gemological research
Gemmological Institute of Thailand (GIT) is closely related to Chulalongkorn University, and has a good reputation for their gemmological research
Asian Institute of Gemmological Sciences (AIGS), the oldest gemological institute in South East Asia, involved in gemological education and gem testing
Swiss Gemmological Institute (SSEF), founded by Prof. Henry Hänni, offering a high scientific standard, and focusing on coloured gemstones and the identification of natural pearls
Gübelin Gem Lab, the traditional Swiss lab founded by the famous Dr. Eduard Gübelin. Their reports are widely considered as the ultimate judgement on high-end pearls, coloured gemstones and diamonds
Each laboratory has its own methodology to evaluate gemstones. Consequently a stone can be called "pink" by one lab while another lab calls it "Padparadscha". One lab can conclude a stone is untreated, while another lab concludes that it is heat treated [6]. To minimise such differences, seven of the most respected labs, i.e. AGTA-GTL (New York), CISGEM (Milano), GAAJ (Tokyo), GIA (Carlsbad), GIT (Bangkok), Gübelin (Lucerne) and SSEF (Basel), have established the Laboratory Manual Harmonisation Committee (LMHC), aiming at the standardisation of wording on reports and certain analytical methods and interpretation of results. Country of origin has sometimes been difficult to find agreement on due to the constant discovery of new locations. Moreover determining a "country of origin" is much more difficult than determining other aspects of a gem (such as cut, clarity etc.) [8].
Gem dealers are aware of the differences between gem laboratories and will make use of the discrepancies to obtain the best possible certificate [6].
Value of gemstones
There are no universally accepted grading systems for any gemstone other than white (colorless) diamond. Diamonds are graded using a system developed by the Gemological Institute of America (GIA) in the early 1950s. Historically all gemstones were graded using the naked eye. The GIA system included a major innovation, the introduction of 10x magnification as the standard for grading clarity. Other gemstones are still graded using the naked eye (assuming 20/20 vision).[2]
For the past several hundred years, gemstones have been broken down into two categories; precious and semi-precious. Though today we think primarily of diamond, ruby, sapphire, and emerald as "precious", these categories are based mainly on fashion and the composition of these two lists has changed frequently over time.[3]
Recently a mnemonic device, the "four C's" (color, cut, clarity and carat), was introduced to help the consumer understand the factors used to grade a diamond.[4] With modification these categories can be useful in understanding the grading of all gemstones. The four criteria carry different weight depending upon whether they are applied to colored gemstones or to colorless diamond. In diamonds, cut is the primary determinant of value followed by clarity and color. Diamonds are meant to sparkle, to break down light into its constituent rainbow colors (dispersion) chop it up into bright little pieces (scintillation) and deliver it to the eye (brilliance). This is a function of cut. In its rough crystalline form, a diamond will do none of these things, it requires proper fashioning and this is called "cut". In gemstones that have color, including colored diamonds, it is the purity and beauty of that color that is the primary determinant of quality.
Physical characteristics that make a colored stone valuable are color, clarity to a lesser extent (emeralds will always have a number of inclusions), cut, unusual optical phenomena within the stone such as color zoning, and asteria (star effects). The Greeks for example greatly valued asteria in gemstones, which were regarded as a powerful love charm, and Helen of Troy was known to have worn star-corundum.[5]
A factor in determining the value of a gemstone is called water. Water is an archaic term that refers to the combination of color and transparency in gemstones; used hierarchically: first water (gem of the finest water), second water, third water, byewater.[6]
Historically gemstones were classified into precious stones and semi-precious stones. Because such a definition can change over time and vary with culture, it has always been a difficult matter to determine what constitutes precious stones.[7]
Aside from the diamond, the ruby, sapphire, emerald, pearl (strictly speaking not a gemstone) and opal [7] have also been considered to be precious. Up to the discoveries of bulk amethyst in Brazil in the 19th century, amethyst was considered a precious stone as well, going back to ancient Greece. Even in the last century certain stones such as aquamarine, peridot and cat's eye have been popular and hence been regarded as precious.
Nowadays such a distinction is no longer made by the trade. [6] Many gemstones are used in even the most expensive jewelry, depending on the brand name of the designer, fashion trends, market supply, treatments etc. Nevertheless, diamonds, rubies, sapphires and emeralds still have a reputation that exceeds those of other gemstones.
Rare or unusual gemstones, generally meant to include those gemstones which occur so infrequently in gem quality that they are scarcely known except to connoisseurs, include andalusite, axinite, cassiterite, clinohumite and bixbite.
Gems prices can fluctuate heavily (such as those of tanzanite over the years) or can be quite stable (such as those of diamonds). In general per carat prices of larger stones are higher than those of smaller stones, but popularity of certain sizes of stone can affect prices. Typically prices can range from 1USD/carat for a normal amethyst to 20,000-50,000USD for a collector's three carat pigeon-blood almost "perfect" ruby.
For the past several hundred years, gemstones have been broken down into two categories; precious and semi-precious. Though today we think primarily of diamond, ruby, sapphire, and emerald as "precious", these categories are based mainly on fashion and the composition of these two lists has changed frequently over time.[3]
Recently a mnemonic device, the "four C's" (color, cut, clarity and carat), was introduced to help the consumer understand the factors used to grade a diamond.[4] With modification these categories can be useful in understanding the grading of all gemstones. The four criteria carry different weight depending upon whether they are applied to colored gemstones or to colorless diamond. In diamonds, cut is the primary determinant of value followed by clarity and color. Diamonds are meant to sparkle, to break down light into its constituent rainbow colors (dispersion) chop it up into bright little pieces (scintillation) and deliver it to the eye (brilliance). This is a function of cut. In its rough crystalline form, a diamond will do none of these things, it requires proper fashioning and this is called "cut". In gemstones that have color, including colored diamonds, it is the purity and beauty of that color that is the primary determinant of quality.
Physical characteristics that make a colored stone valuable are color, clarity to a lesser extent (emeralds will always have a number of inclusions), cut, unusual optical phenomena within the stone such as color zoning, and asteria (star effects). The Greeks for example greatly valued asteria in gemstones, which were regarded as a powerful love charm, and Helen of Troy was known to have worn star-corundum.[5]
A factor in determining the value of a gemstone is called water. Water is an archaic term that refers to the combination of color and transparency in gemstones; used hierarchically: first water (gem of the finest water), second water, third water, byewater.[6]
Historically gemstones were classified into precious stones and semi-precious stones. Because such a definition can change over time and vary with culture, it has always been a difficult matter to determine what constitutes precious stones.[7]
Aside from the diamond, the ruby, sapphire, emerald, pearl (strictly speaking not a gemstone) and opal [7] have also been considered to be precious. Up to the discoveries of bulk amethyst in Brazil in the 19th century, amethyst was considered a precious stone as well, going back to ancient Greece. Even in the last century certain stones such as aquamarine, peridot and cat's eye have been popular and hence been regarded as precious.
Nowadays such a distinction is no longer made by the trade. [6] Many gemstones are used in even the most expensive jewelry, depending on the brand name of the designer, fashion trends, market supply, treatments etc. Nevertheless, diamonds, rubies, sapphires and emeralds still have a reputation that exceeds those of other gemstones.
Rare or unusual gemstones, generally meant to include those gemstones which occur so infrequently in gem quality that they are scarcely known except to connoisseurs, include andalusite, axinite, cassiterite, clinohumite and bixbite.
Gems prices can fluctuate heavily (such as those of tanzanite over the years) or can be quite stable (such as those of diamonds). In general per carat prices of larger stones are higher than those of smaller stones, but popularity of certain sizes of stone can affect prices. Typically prices can range from 1USD/carat for a normal amethyst to 20,000-50,000USD for a collector's three carat pigeon-blood almost "perfect" ruby.
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