A pearl is a hard, lustrous rounded concretion produced by certain animals, primarily mollusks such as oysters.
A pearl is formed when a foreign material, usually a sharp object such as a fragment of rock, a sand grain or parasite enters the oyster or clam and cannot be expelled. This irritates the mollusk and to reduce irritation, the mollusk coats the intruder with the same secretion it uses for shell-building, i.e. nacre. This results in coating the foreign material with layer upon layer of shell material. In this way a pearl is formed.
Looking at the chemistry of the pearls so formed inside the shell of certain bivalve mollusks, we find layers of calcium carbonate (CaCO3) in the form of the minerals aragonite or calcite (both crystalline forms of calcium carbonate) held together by an organic horn-like compound called conchiolin. This combination of calcium carbonate and conchiolin is called nacre, or as we more commonly know it, mother-of-pearl. The unique luster of pearls is a result of the reflection and refraction of light from its translucent layers and is finer in proportion as the layers become thinner and more numerous. The iridescence that some pearls display is caused by the overlapping of successive layers, which breaks up the light falling on the surface.
Pearls are the most valued among all the gemstones because of their delicate translucence and luster. White is the most commonly found color, though pearls are also found in delicate shades of pink, black, cream, gray , blue, yellow, lavender, green and mauve. Black pearls can be fond in the Gulf of Mexico and in waters off some islands in the Pacific Ocean. The Persian Gulf and Sri Lanka are well known for exquisite cream colored pearls called Orientals. Natural seawater pearls are also found in the waters off Celebes in Indonesia, the Gulf of California and the Pacific Coast of Mexico. The Mississippi river and forest streams of Bavaria also produce pearls.
Pearls are formed at times on the inside of the shells of the mollusk. These are usually irregular in shape and have little commercial value. However, those formed within the tissue of the mollusk are either spherical or pear-shaped, and are highly valued for use as jewelry. Besides being used in jewelry pearls are also crushed and used in cosmetics or paint. Pearl is highly valued as a gemstone and is therefore cultivated or harvested for jewellery. The pearl is also the birthstone of June.
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