Pearls

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Pearl

Pearls are created inside of bivalve molluscs, mainly oysters and mussels, through the accidental or deliberate introduction of irritant deposit such as a grain of sand. The lustrous Pearl is made of a substance called nacre which comprises mainly of calcium carbonate as aragonite crystal combined with a horn substance known as conchiolin that binds the microcrystals concentrically around the irritant. Although pearls are not especially hard, only about 2.5 to 4.5 on the Mohs scale, they are extraordinarily compact and very difficult to crush.

The typical pearly luster is produced by the overlapping platelets of aragonite and film of conchiolin nearest to the pearl surface. The colour of pearl will be determined according to the type of mollusc and the water in which it grows. Although the pearl will be coloured right through the final colour will be determined by the upper conchiolin layer.
Pearls were once upon a time found only in nature and were hence deemed valuable. Nowadays you will find an abundance of affordable pearls which have been cultured.
Natural or cultured pearls may occur in the sea or in freshwater.