Pearls are unique among gemstones because they hold the distinction of being the only gem formed inside a living species. Both natural and cultured pearls are grown in certain types of mollusks that can live in either saltwater or freshwater.
Natural pearls are formed when a foreign particle such as a grain of sand, enters the shell and irritates the mollusk. In response, the mollusk covers the irritant with sheets of nacre, a combination of calcium carbonate and conchiolin. The secretion process, continues as long as the mollusk lives, eventually forms the pearl.
Natural pearls are rare today because the pearl-producing species of mollusks are near extinction due to excessive harvesting in the 18th and 19th centuries. Most natural pearls on today's market are vintage pearls and heirloom pieces. They are quite expensive and usually sold by carat weight. Since the likelihood of finding pearls in the wild is very slim, virtually all pearls used in jewelry now are cultured.
Cultured pearls are real, genuine pearls that are formed inside of a living oyster with human intervention. The majority of cultured pearls available today are derived from freshwater mussels and saltwater pearl oysters. There are two basic types of farmed pearls: bead nucleated and mantle nucleated.
Saltwater pearls are bead nucleated. A polished bead made from a mussel shell along with a tiny graft of mantle tissue from an oyster is inserted in the oyster's reproductive organ. Using this method helps stimulate nacre production and the spherical bead serves as the mold, around which concentric layers of nacre are secreted. This process produces a classic round pearl shape.
The culturing process usually takes place over a period of one to two years, depending on the species producing the cultured pearls. Among the most popular pearls, Akoya take the least amount of time to culture and harvest, from 8 months to 2 years. More exotic cultured pearls, such as South Sea Pearls and Tahitian Pearls take about 2 to 6 years.
While saltwater pearls are beaded, freshwater pearls are typically mantle-nucleated, meaning they are made entirely of nacre with no bead inside. A skilled technician inserts a tissue fragment into the host mussel's mantle. Nacre forms around it to produce the pearl. As many as 10 to 50 pearls can be cultured in one mussel. Most freshwater pearls are round to off round. Freshwater pearls range from 3 mm to 12 mm and are usually smaller than saltwater pearls, which can range from 8 to 18 mm.
A pearl’s value depends on the seven value factors defined by GIA: size, shape, color, luster, surface, nacre, and matching. Shapes range in descending order of value from round to semi-round, off-round, oval, drop, and baroque. Shapes from round to drop are symmetrical, whereas baroque denotes a pearl that is irregular in form. In addition to shape, a pearl's value is dependent on the quality and thickness of the nacre affects all other aspects of a pearl's quality, especially its life span and luster because pearls with thinner layers of nacre will deteriorate much more quickly.