What type of shell contains a pearl?

A pearl is typically found within the shells of mollusks, particularly oysters and certain types of clams. The most renowned source of pearls is the saltwater oyster from the family Pteriidae, especially species like the Akoya oyster, the Tahitian black-lipped oyster, and the South Sea oyster. These mollusks produce pearls as a defense mechanism against irritants, forming layers of nacre around the foreign object.