Pearl necklace. Pearls are accretions of nacre (calcium carbonate) secreted by certain oysters and mussels. The presence of a foreign body (such as a parasite or a grain of sand) within the shell triggers the secretion of nacre by the mollusc's mantle. Thin layers are deposited on the irritant particle and, over several years, a pearl is formed. Although commercially valuable pearls are obtained from freshwater mussels and oysters, precious pearls can come from ocean oysters. Cultured pearls are cultivated by inserting fragments of clam shell into the shells of farmed mussels and oysters.

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