Natural pearls are formed more or less randomly, when some sort of irritant becomes lodged in the tissue of an oyster or mollusk. In response to the irritation, the oyster secretes nacre*, a combination of calcium carbonate and organic substances, which gradually builds up in layers around the irritant. Over a period of several years, this build-up of nacre forms a pearl.

The size, shape, and color of the pearl are determined by a combination of factors, including the size and shape of the original irritant, whether the mollusk is living in salt or freshwater, and the geographic region where the mollusk lives.

Natural pearls of any commercial value or desirability are extremely rare. Instead, since the early part of the 20th century, cultured pearls have supplanted natural pearls as the most common and available pearls.

Cultured pearls are still actual pearls, grown organically inside of oysters in the same way as natural pearls. The difference is that, in the case of cultured pearls, the pearl farmer intentionally stimulates the development of the pearl by inserting a "nucleus" into the oyster. Thus, the formation and discovery of the pearl are no longer left to chance.

Natural pearls today tend to be found primarily in older jewelry from estate sales, auctions, and so forth -- in other words, existing pearls rather than new ones. However, some natural pearl beds are being increasingly harvested, including beds in the Persian Gulf area and freshwater natural pearls in the United States.