In your hand is a string of pearls. They look real enough, reflecting the light with a magnificent luster, but how can you be sure they aren’t fakes? Simply rub a pearl lightly along your front teeth. If it feels rough or gritty, it’s real. If it feels smooth, it’s fake. How come? Real pearls are created by a number of fresh- and salt-water members of the bivalve mollusk class, although the pearls used for jewelry generally come from pearl oysters. Pearls begin as an irritating particle inside an oyster’s shell, a grain of sand perhaps. The oyster coats this irritant with a substance called “nacre” (pronounced NAY-ker), the same iridescent material that lines the inside of its shell. Inside a shell, this nacre coating is relatively thin, and it’s called “mother-of-pearl.” Once the oyster starts wrapping this mother of pearl around an irritant though, it doesn’t know when to stop. It adds layer after layer of nacre, until a full-sized pearl is formed.