A pearl is a hard object produced within the soft tissue (specifically the mantle) of a living shelled mollusk. 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.
The most valuable pearls occur spontaneously in the wild, but they are extremely rare. These wild pearls are referred to as natural pearls. Cultured or farmed pearls from pearl oysters and freshwater mussels make up the majority of those that are currently sold. Imitation or fake pearls are also widely sold in inexpensive jewelry, but the quality of their iridescence is usually very poor, and generally speaking, artificial pearls are easily distinguished from genuine pearls. Pearls have also been crushed and used in cosmetics, medicines, and in paint formulations.
Whether wild or cultured, gem quality pearls are almost always nacreous and iridescent, as is the interior of the shell that produces them. However, almost all species of shelled mollusks are capable of producing pearls (formerly referred to as "calcareous concretions" by some sources) of lesser shine or less spherical shape.