The inside shells of oysters and other shell-forming mollusks are covered with a shiny, lustrous substance called nacre, or mother-of-pearl. Only tropical sea pearl oysters have the beautifully colored nacre necessary to make valuable pearls. Other edible clams and oysters also make pearls. But we would not recognize them as pearls, and they have no value.
When a foreign particle such as a piece of sand or a parasite enters the oyster’s shell, these nacre-producing cells set to work, covering the particle with layer upon layer of the nacre substance, until the particle is completely enclosed.
The color of the pearl will be the same as the nacre lining of the shell. That is why there are black (really a dark gray), pink, cream-colored, and white pearls.
It takes a pearl-producing oyster about seven years to make a moderate-sized pearl!