Pearls are formed inside the shell of certain bivalve mollusks, i.e. mollusks which shell is made of two more or less symmetrical parts (valves). The valves are kept open for most of the time so the oyster can eat.

The formation of a pearl begins when a foreign substance enters the shell. In response to such irritation the mollusk begins covering the irritant with the layers of nacre (mother-of-pearl), the material which lines the inside of the shell.

A good sized pearl can take between three to six years to form, which is usually the entire life of the oyster or mollusk.

The process of creating the cultured pearls was first developed by Kokichi Mikimoto in Japan, and patented in 1896. Cultured pearls are created by the same process as natural pearls, but the irritants are placed in the oysters by the pearl harvesters.

The different species produce different types of pearls. Black South Sea Pearls, or Tahitian pearls come from the Black-lip oyster; White and Golden South Sea pearls – from the White-lip and Golden-lip oysters; and Akoya cultured pearls – from the Japanese pearl oyster.

Clams and mussels can also produce pearls, but that is a much rarer occurrence.

The History of Pearls