A pearl is a hard object produced within the soft tissue (specifically the mantle) of a living shelled mollusk or another animal, such as a conulariid. Just like the shell of a mollusk, a pearl is composed of calcium carbonate (mainly aragonite or a mixture of aragonite and calcite) in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes, known as baroque pearls, can 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. The most valuable pearls occur spontaneously in the wild, but are extremely rare. These wild pearls are referred to as natural pearls.