A pearl is a hard, shiny object produced within the soft tissue (particularly the mantle) of a living mollusk with a shell or other animal, such as fossil conules. 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 is deposited in concentric layers. The ideal pearl is perfectly round and smooth, but there are many other shapes known as baroque pearls. The best quality of natural pearls have been regarded as precious stones 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 called natural pearls. Cultured or cultured pearls from pearl oysters and freshwater mussels make up the majority of those currently sold. Imitation pearls are also widely sold in cheap jewelry, but the quality of their iridescence is usually very poor and is easily distinguished 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 up 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 mollusks are capable of producing pearls (technically “calcareous concretions”) of less luster or less spherical shape. Although these can also be legitimately called “pearls” by gemological laboratories and also under the rules of the Federal Trade Commission of the United States,  and are formed in the same way, most of them have no more value than curiosity.
All shelled mollusks can, by natural processes, produce a kind of “pearl” when an irritating microscopic object gets caught in the folds of their mantle, but the vast majority of these “pearls” are not valued as gemstones. Nacreous pearls, the best known and most commercially important, are produced mainly by two groups of bivalve mollusks or clams. A nacre pearl is made up of layers of nacre, with 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 pearl oysters or mussels have to be collected and cut open and then slaughtered to find even one wild pearl; For many centuries this has been the only way to obtain pearls and why pearls have fetched such extraordinary prices in the past. Cultured pearls are formed in pearl farms, through 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 sea oysters in the Pteriidae family. Freshwater pearls grow within some (but not all) species of freshwater mussels in the order Unionida, the families Unionidae and Margaritiferidae.
The unique luster of pearls depends on the reflection, refraction, and diffraction of light from the translucent layers. The finer and more numerous the layers of the pearl, the finer the luster. The iridescence manifested by the pearls is due to the superposition of successive layers, which breaks the light that falls on the surface. Also, pearls (especially freshwater cultured pearls) can be dyed yellow, green, blue, brown, pink, purple, or black. The best pearls have a mirror-like metallic luster.
Since pearls are mainly made up of calcium carbonate, they can be dissolved in vinegar. Calcium carbonate is also susceptible to a weak acidic solution because the crystals react with the acetic acid in the vinegar to form calcium acetate and carbon dioxide.