A pearl is a hard, rounded object produced within the soft tissue (specifically the mantle) of a living shelled mollusk. Just like the shell of mollusks, a pearl is composed of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes of pearls occur, see baroque pearl.

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.

Biologically speaking, under the right set of circumstances, almost any shelled mollusk can produce some kind of "pearl", however, most of these 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 are sometimes not even very durable, such that they usually have no value at all, except perhaps to a scientist, or as a curiosity. These objects would be referred to as "calcareous concretions" by a gemologist, even though a malacologist would still consider them to be pearls.

One unusual example of "calcareous concretions" which nonetheless can sometimes have value, are the "pearls" which very rarely are found growing between the mantle and the shell of the queen conch or pink conch, Strombus gigas, a large sea snail or marine gastropod from the Caribbean Sea. These "pearls" are a by-product of the conch fishing industry, and the best of them show some chatoyance.

However, true iridescent pearls, the most desirable pearls, are all produced by two very different groups of molluscan bivalves or clams. One group lives in the sea: the pearl oysters. The other, very different group of bivalves live in freshwater; these are the freshwater or river mussels.

Saltwater pearls can grow in several species of marine pearl oysters in the family Pteriidae. Freshwater pearls grow within certain (but by no means all) species of freshwater mussels in the order Unionida, the families Unionidae and Margaritiferidae. All of these bivalves are able to make true pearls because they have a thick inner shell layer composed of "mother of pearl" or nacre. The mantle of the living bivalve can create a pearl in the same way that it creates the pearly inner layer of the shell.

Fine gem quality saltwater and freshwater pearls can and do sometimes occur completely naturally, but this is a rare occurrence. Many hundreds of pearl oysters or pearl mussels have to be gathered and opened (killed) in order to find even one pearl, and for many centuries that was the only way pearls were obtained. This was the main reason why pearls fetched such extraordinary prices in the past. In modern times however, almost all the pearls for sale were formed with a good deal of expert intervention from human pearl farmers.

A true pearl is made from layers of nacre, by the same living process as is used in the secretion of the mother of pearl which lines the shell. A "natural pearl" is one that formed without any human intervention at all, in the wild, and these are very rare. A "cultured pearl" on the other hand, is one that has been formed on a pearl farm. The great majority of pearls on the market are cultured pearls.

Imitation or fake pearls are also widely sold in inexpensive jewelry, but the quality of the iridescence is usually very poor, and generally speaking, fake pearls are usually quite easy to distinguish from the real thing.

True pearls have long been greatly valued as gemstones. They have been harvested, or more recently cultivated, primarily for use in jewelry but were also historically worn stitched onto lavish clothing as worn, for example, by royalty. Pearls have also been crushed and used in cosmetics, medicines, or in paint formulations.

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