A pearl is a hard, shiny object produced within the soft tissue (especially the mantle) of a living mollusk with a shell or other animal, such as fossil conulars. 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 can appear, known as baroque pearls. The best quality of natural pearls has been highly regarded as gemstones and objects of beauty for many centuries. For this reason, the pearl has become a metaphor for something rare, refined, admirable and precious.
The most precious pearls are found spontaneously in nature, but they are extremely rare. These wild pearls are known as natural pearls. Cultured pearls or pearl cultured oysters and freshwater mussels make up the majority of those currently sold. Faux pearls are also widely sold in inexpensive jewelry, but the quality of their iridescence is often very poor and easily distinguishable from that of genuine pearls. Pearls were collected and cultivated primarily for use in jewelry, but in the past they were also used to adorn clothing. They have also been ground and used in cosmetics, medicines, and paint formulations.
Whether wild or cultured, gem-quality pearls are almost always pearlescent and iridescent, like the inside of the shell that produces them. However, almost all species of shell molluscs are capable of producing pearls (technically “calcareous concretions”) of less brilliance or less spherical shape. Although these may also legitimately be called “pearls” by gemological laboratories and also according to the rules of the United States Federal Trade Commission,  and are formed in the same way, most of them are of no value other than curiosity.
All shelled mollusks can, through natural processes, produce some kind of “pearl” when an irritating microscopic object becomes trapped in the folds of their mantle, but the vast majority of these “pearls” are not valued as precious stones. Pearl pearls, the best known and most commercially important, are mainly produced by two groups of bivalve molluscs or clams. A nacreous pearl is made up of layers of nacre, through the same living process that is used in the secretion of the nacre that covers the shell.
Natural (or wild) pearls, formed without human intervention, are very rare. Many hundreds of oysters or mussels have to be harvested and cut, and then killed, to find even a wild pearl; For many centuries, this was the only way pearls were obtained and why pearls have achieved such extraordinary prices in the past. Cultured pearls are formed in pearl farms, using human intervention and natural processes.
One family of pearly bivalves, the pearl oyster, lives in the sea, while the other, a very different group of bivalves, lives in fresh water; These are river mussels like the freshwater pearl mussel. Saltwater pearls can grow on several species of marine oysters of the Pteriidae family. Freshwater pearls grow within some (but not all) species of freshwater mussels of the order Unionida, of the Unionidae and Margaritiferidae families.