A class of sea creatures in the mollusk family, known as bivalves, are the source of pearls. This group of marine animals includes clams, mussels and oysters. Every bivalve species has the ability to produce pearls.
Pearls form inside of certain mollusks as a defense mechanism. When a foreign object gets stuck inside of an oyster, for example, the oyster responds by coating the object with two proteins. This protein layer makes the object less painful to the creature. Over time, the oyster covers the object with several more layers of protein, and this successive layering ultimately creates a pearl.