How do oysters produce pearls?

Pearls begin with the presence of a foreign substance, such as a grain of sand, that lodges in the shell. The oyster's body reacts by depositing layers of nacreous (pearl-like) material around the foreign body to wall it off and reduce irritation. Many oysters--as well as some clams and mussels--manufacture material like the pearl- producing substance. True pearl-producing oysters, however, inhabit waters of the Indo Pacific.