Pearls are precious stones formed inside shellfish such as oysters, mussels and clams. They form when a piece of grit enters the creature’s shell. The most valuable pearls are those from oysters.
Pearl is a valuable gem known to mankind since ancient times. The pearl, in fact, is of animal origin and produced by certain bivalves of Mollusca. The pearl producing bivalves are marine oysters of the genus Pinctada, though some freshwater bivalves of the genus Unio and Anodonta also produce pearl but of inferior quality and rarely of any use.
The pearl is secreted by the mantle as a protective measure against foreign objects like sand particles, parasites, small larvae or any object of organic and inorganic origin. In fact, as soon as a foreign object, somehow, enters the body of a bivalve in between the shell and mantle, the mantle immediately gets irritated and at once encloses it like a sac. The mantle wall then starts secreting layers of nacre around the foreign object from defence point of view.
Thus, mantle wall secretes continuously several layers of nacre around the foreign object and finally pearl is formed. The value of pearl depends upon its size, quality, etc. Now a day, the pearl producing bivalves are reared and pearls are produced artificially by introducing some foreign objects between the mantle and shell in the different parts of the world; Japan has surpassed all other countries in this field.
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