A
pearl is a hard object produced within the soft tissue (specifically the
mantle) of a living
shelled
mollusc. Just like the shell of a clam, a pearl is composed of
calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes (
baroque pearls) occur. The finest quality natural pearls have been highly valued as
gemstones and objects of
beauty for many centuries. Because of this,
pearl has become a
metaphor for something rare, fine, admirable and valuable.