A pearl is built up from thin, thin layers of nacre, the glossy material which the oyster uses to line the inside of his shell. This is a long and slow job, for the material must be extracted from sea water and processed by special cells in the oyster's body. Sometimes it takes a whole year to add just a few layers to the shell walls and the growing pearl.
The pearl forms around a grain of sand, a small worm or some other fragment which has found its way into the oysterfs shell. In a cultured pearl, a core is placed inside the shell and it takes the oyster two or three years to cover it with a thin layer of nacre. After five to seven years, the nacre is thicker, making the pearl bigger and more valuable,
A precious natural pearl may take twenty years or more to form inside the oyster.