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.