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