Most of us know that pearls come from oysters, those shelled mollusks of the sea. (Pedantically, this isn’t quite true; the oysters we eat can create pearls, but most jewelry pearls actually come from other species of mollusks that are not oysters. I may still refer to these creatures as oysters throughout this article, however, as oysters are capable of producing pearls, even though they aren’t jewelry quality.)
Natural pearls form when a bit of sand or some other hard irritant gets inside the shell of a mollusk. In order to alleviate the rubbing of this irritant against the mollusk’s soft tissues, the creature coats it in layers of a substance called nacre. We sometimes call this substance mother-of-pearl.