Pearls: Nature's precious gem

by Lisa Cardy

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The oyster eases the irritant by secreting a smooth crystalline substance, called 'nacre' around it. Nacre is composed of an organic, horn-like compound called conchiolin, and calcium carbonate (CaCO3).

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Over time, nacre is secreted in a succession of concentric, very thin, translucent layers around the irritant. Nacre contains aragonite, which is laid down perpendicular to the nacre. This arrangement of aragonite and nacre is highly regular and causes the pearls iridescence and unique lustre.

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Naturally grown pearls are usually found in sea water. Impurities in the water in which the animal lives can produce yellowish, pinkish, reddish, or blackish-grey pearls.

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The shape of a pearl depends on the shape of the original irritant and whether the pearl moves during its growth. For these reasons, most natural pearls are not perfectly round.

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The rate of deposition of nacre to form a pearl is very slow, only 0.6mm per year. And, given that natural pearls form in only about 1 in 40 oysters, large natural pearls are understandably rare.

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The Chinese first started culturing pearls back in the thirteenth century and the Japanese perfected the technique by the twentieth century. Cultured pearls are formed in almost an identical manner as natural pearls. The difference being that the irritant, normally a small piece of polished shell, is implanted. It takes about three years to produce a cultured pearl.

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A natural pearl is pearl all the way through. Whereas a cultured pearl is mainly a shell bead with a very thin pearl coating.

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Pearls are normally associated with oysters, and the vast bulk of commercial pearls come from the 'pearl oyster'. But all species of bivalves (clams, oysters, mussels) are capable of producing pearls.

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However, mussel pearls, unlike those that are formed by pearl oysters, are usually smaller and of poor colour and are therefore not as valuable.

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Cultured pearls are grown both in salt water and fresh water. Oysters are usually used in salt water, and mussels are usually used in fresh water.

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