Pearls occur naturally when certain mollusks are infected with parasitic organisms or other such irritants, usually when they burrow through the shell into the tissue underneath. The mollusk’s immune system triggers the secretion of a mucus like substance called nacre, which coats the irritant to protect the mollusk from damage. Over time, the layers of nacre coating build up, resulting in the formation of a pearl within the shell. Pearls tend to retain the shape of the original irritant, and so most natural pearls are not round.

Naturally occurring pearls are rare, and many thousands of mollusks can be killed in the search for one pearl. This is why natural pearls command the highest prices, as the yield is unpredictable. As pearls are so desirable and so rare, pearl farmers have worked out ways to stimulate the pearl formation process, greatly increasing the yield of pearls. Originating in Japan in the early 20th century, pearl farming involves the artificial introduction of an irritant to the pearl yielding mollusk, followed by its return to its aquatic habitat. The pearls are then given between 2 and 6 years to grow, depending on the size of pearl required, before harvest. Each mollusk can produce up to 32 pearls.

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