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 made up 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 of pearls (baroque pearls)
occur. The finest quality natural pearls have been highly valued as
gemstones and objects of beauty for many centuries, and because of this,
the word pearl has become a metaphor for something very rare, fine,
admirable, and valuable.