Chemical composition -- Organic gem material, primarily
calcium carbonate (calcite ). Mother
of pearl lines the inner surface of the shell of a mollusc
(e.g., oyster). A pearl forms in layers around an irritating foreign
body within a mollusc.
Color --White, black, shades of other colors.
Optics -- R.I. 1.53-1.69.
Durability -- Hardness 2.5-4.5.
Crystal structure -- Orthorhombic. Pearls have a distinctive concentric structure.
Specific Gravity-- 2.6-2.8.
Sources -- Salt-water sources include the Persian Gulf, Indian Ocean, Australia, South Seas, and Japan. Fresh-water sources include major rivers such as the Mississippi and Amazon River basins. Japan has perfected the art of creating cultured pearls, wherein foreign bodies are deliberately introduced into oysters and harvested after several years. Fine natural salt-water pearls were highly prized in the early 20th century, but the introduction of cultured pearls has largely displaced them in the market.