HOW PEARLS ARE FORMED
Nearly all shelled mollusks can, by natural means, create some type of "pearl" when a microscopic object chafes the mollusk and becomes caught inside the mollusk's mantle folds, but the biggest portion of these "pearls" are not valuable at all.
Nacreous pearls, the most recognizable and commercially-important pearls, are chiefly created by two groups of molluscan bivalves or clams. A nacreous pearl is formed from layers of nacre via the same living process utilized in the secretion of the mother of pearl that outlines the shell.
NATURAL vs CULTURED
A "natural pearl" forms naturally, without any human intervention necessary, and is extremelly rare. Several hundred pearl oysters or mussels must be collected and opened, killing them, in the hope of discovering even one natural pearl, and in times past this was the only way pearls were able to be recovered, and thusly why they were so expensive. In contrast, a cultured pearl is one fashioned using human involvement on a pearl farm. The bulk of today's pearls are technically cultured pearls.
SALT & FRESH WATER
One family of nacreous pearl bivalves, the pearl oyster, makes its home in the sea while the other, dissimilar familly of bivalves must live in freshwater, such as the river mussel (fresh water pearl mussel). Saltwater pearls can develop in more than a few species of marine pearl oysters in the genus Pteriidae. Freshwawter pearls develop inside particular (but not nearly all) varieties of freshwater mussels in the species Unionida, Uniondae, and Margaritiferidae.