Pearl

Pearl
Various pearls
General
Category	Carbonate mineral, protein
Formula (repeating unit)	Calcium carbonate, CaCO3 Conchiolin
Strunz classification	05.8
Crystal system	Orthorhombic[1]
Identification
Color	White, pink, silver, cream, brown, green, blue, black, yellow, orange, red, gold, purple, iridescent
Cleavage	None[1]
Fracture	Uneven, various
Mohs scale hardness	2.5–4.5[1]
Streak	White
Specific gravity	2.60–2.85[1]
Refractive index	Common pearl: 1.52-1.66 Black pearl: 1.53-1.69[1]
Birefringence	0.156
Pleochroism	Absent
Dispersion	None
Ultraviolet fluorescence	White pearls: light blue to light yellow; Yellow and golden pearls: yellow-green, greenish brown to dark brown; Black pearls: commonly pink to orange-red[2]

A pearl is a hard, glistening object produced within the soft tissue (specifically the mantle) of a living shelled mollusk or another animal, such as fossil conulariids. Just like the shell of a mollusk, a pearl is composed of calcium carbonate (mainly aragonite or a mixture of aragonite and calcite)^[3] in minute crystalline form, which has deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes, known as baroque pearls, can occur. The finest quality of natural pearls have been highly valued as gemstones and objects of beauty for many centuries. Because of this, pearl has become a metaphor for something rare, fine, admirable and valuable.

The most valuable pearls occur spontaneously in the wild, but are extremely rare. These wild pearls are referred to as natural pearls. Cultured or farmed pearls from pearl oysters and freshwater mussels make up the majority of those currently sold. Imitation pearls are also widely sold in inexpensive jewelry. Pearls have been harvested and cultivated primarily for use in jewelry, but in the past were also used to adorn clothing. They have also been crushed and used in cosmetics, medicines and paint formulations.

Whether wild or cultured, gem-quality pearls are almost always nacreous and iridescent, like the interior of the shell that produces them. However, almost all species of shelled mollusks are capable of producing pearls (technically "calcareous concretions") of lesser shine or less spherical shape. Although these may also be legitimately referred to as "pearls" by gemological labs and also under U.S. Federal Trade Commission rules,^[4] and are formed in the same way, most of them have no value except as curiosities.