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natural but cultivated pearl produced by a mollusk after the intentional introduction of a foreign object inside the creature's shell. The discovery that such pearls could be cultivated in freshwater mussels is said to have been made in 13th-century China, and the Chinese have been adept for hundreds of years at cultivating pearls by opening the mussel's shell and inserting into it small pellets of mud or tiny bosses of wood, bone, or metal and returning the mussel to its bed for about three years to await the maturation of a pearl formation. Cultured pearls of China have been almost exclusively blister pearls (hemispherical pearls formed between the mussel and its shell), which require addition of a half sphere of mother-of-pearl to create the assembled gem, called a pearl doublet.