Pearl jewelry is mainly defined by two parties, freshwater pearl jewelry and cultured pearls jewelry .

A cultured pearl is a real pearl jewelry, made from the same substance as a natural pearl jewelry. The difference: Cultured pearls get their start with a little nudge. Rather than waiting for nature to take its course, man deliberately places an irritant—a mother-of-pearl jewelry bead is often used—in the mollusk to stimulate the secretion of aragonite (calcium carbonate) and conchiolin. These substances, when layered around the irritant, form a pearl jewelry. Today, cultured pearls can rival the beauty of natural pearls; indeed it can be very difficult to tell them apart.

A freshwater pearl is a pearl jewelry that has been formed in a freshwater mollusk, rather than an oyster that lives in the sea. Typically, people think freshwater pearls are rice-shaped, but in fact they come in all shapes, including round. Like marine pearls, round freshwater pearls are the rarest, most desirable and most valuable shape. It is difficult to find perfectly round pearls jewelry of similar size and color to match for a strand, which is why matched strands are more expensive than other types of pearl jewelry. Unlike oysters, freshwater mollusks can produce more than one pearl jewelry at a time, but the process is still at Mother Nature's mercy. Out of the thousands of animals that are implanted with irritants, it is still rare to get a commercially acceptable pearl.
A pearl jewelry 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 jewelry 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 jewelry 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.

Almost any shelled mollusk can, by natural processes, produce some kind of "pearl" when an irritating microscopic object becomes trapped within the mollusk's mantle folds, but virtually none of these "pearls" are considered to be gemstones.

True iridescent pearls jewelry, the most desirable pearls jewelry, are produced by two groups of molluscan bivalves or clams. One family lives in the sea: the pearl oysters. The other, very different group of bivalves live in freshwater, and these are the river mussels, for example, see the freshwater pearl mussel.

Saltwater pearls jewelry can grow in several species of marine pearl oysters in the family Pteriidae. Freshwater pearls jewelry 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 jewelry 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 jewelry 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 jewelry were obtained. This was the main reason why pearls jewelry fetched such extraordinary prices in the past. In modern times however, almost all the pearls jewelry 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 jewelry on the market are cultured pearls jewelry.

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

Pearls have been harvested, or more recently cultivated, primarily for use in jewelry, but in the past they were also stitched onto lavish clothing, as worn, for example, by royalty. Pearls jewelry have also been crushed and used in cosmetics, medicines, or in paint formulations.