A pearl is a hard, rounded object produced within the soft tissue (specifically the mantle) of a living shelled mollusk. The finest quality pearls have been highly valued as gemstones and objects of beauty for many centuries, and the word pearl has become a metaphor for something rare, fine, and admirable.Technically speaking, under the right set of circumstances almost any shelled mollusk can produce some kind of “pearl”. However, most molluscan “pearls” have no luster or iridescence. In fact, the great majority of mollusk species produce pearls which are not attractive to look at, and not durable, such that they usually have no value at all, except perhaps to a scientist, or as a curiosity.Desirable pearls are produced by two very different groups of molluscan bivalves or clams: marine pearl oysters from the family Pteriidae and freshwater pearl mussels from the order Unionida, families Unionidae and Margaritiferidae. These two families of clams, although not closely related to one another, have shell linings that are composed of nacre, or “mother of pearl” as it is more commonly known. A natural pearl is entirely made from layers of nacre, using the same spontaneous process as is used in the creation of the nacre lining the shell.One other kind of gemstone-quality pearl is created by a large sea snail or marine gastropod. These large, deep pink pearls are not very “pearly” although they can have a good luster. They grow within the mantle of the queen conch or pink conch, Strombus gigas from the Caribbean. These conch pearls occur naturally, although they are very rare. They are a side product of the conch fishery as food production.Although in illustrations a gem-quality pearl is often pictured sitting in an edible oyster shell, this is a mistaken idea. Pearl oysters are not closely related to edible oysters, and edible oysters cannot form real pearls.Pearls have long been greatly valued as gemstones. They have been harvested, or more recently cultivated, primarily for use in jewelry. Pearls have also, rarely, been crushed and used in cosmetics or in paint formulations.Pearl is considered to be the birthstone for the spring/summer month of June.