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].

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.

Characteristics and classification

Gemstones are identified by gemologists, who describe gems and their characteristics using technical terminology specific to the field of gemology. The first characteristic a gemologist uses to identify a gemstone is its chemical composition. For example, diamonds are made of carbon (C) and rubies of aluminium oxide (Al2O3). Next, many gems are crystals which are classified by their crystal system such as cubic or trigonal or monoclinic. Another term used is habit, the form the gem is usually found in. For example diamonds, which have a cubic crystal system, are often found as octahedrons.




Gemstones are classified into different groups, species, and varieties. For example, ruby is the red variety of the species corundum, while any other color of corundum is considered sapphire. Emerald (green), aquamarine (blue), bixbite (red), goshenite (colorless), heliodor (yellow), and morganite (pink) are all varieties of the mineral species beryl.

Gems are characterized in terms of refractive index, dispersion, specific gravity, hardness, cleavage, fracture, and lustre. They may exhibit pleochroism or double refraction. They may have luminescence and a distinctive absorption spectrum.

Material or flaws within a stone may be present as inclusions.

Definition Of GEMS

Geology: Rock (geology), a naturally occurring aggregate of minerals and/or mineraloids, or an individual piece of rock
Marble, a metamorphic rock resulting from the metamorphism of limestone

GEMSTONE:

A gemstone or gem, also called a precious or semi-precious stone, is a piece of attractive mineral, which — when cut and polished — is used to make jewelry or other adornments.[1] However certain rocks, (such as lapis-lazuli) and organic materials (such as amber or jet) are not minerals, but are still used for jewelry, and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their lustre or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone.

Units of measure:

Stone (mass), a unit of mass which since the latter half of the twentieth century has been equal to fourteen pounds, i.e. 6.35 kilograms
Stone (Chinese weight), a Chinese unit of weight equal to 120 piculs
or 160 pounds.