A smooth lustrous round structure inside the shell of a clam or oyster; much valued as a jewel WordNet 3.0 & Open English WordNet

A pearl is a hard, glistening object produced within the soft tissue of a living shelled mollusk or another animal, such as fossil conulariids. Wikipedia

Hard object produced by mollusks. Wikipedia Disambiguation

Hard object produced within the soft tissue of a living shelled mollusc Wikidata

A hard, rounded object produced within the soft tissue (specifically the mantle) of a living shelled mollusk. OmegaWiki

A shelly concretion, usually rounded, and having a brilliant luster, with varying tints, found in the mantle, or between the mantle and shell, of certain bivalve mollusks, especially in the pearl oysters and river mussels, and sometimes in certain univalves. It is usually due to a secretion of shelly substance around some irritating foreign particle. Its substance is the same as nacre, or mother-of-pearl. Round lustrous pearls are used in jewellery. Wiktionary