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 v Fake
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.
At A Microscopic Level
Pearls are rough because nacre is formed of hard, irregular crystals. At a microscopic level, these crystals make a series of irregular plates and ridges on a pearl's surface.
This crystalline structure, which gives a pearl its iridescent properties, is also responsible for its gritty texture.
Fake pearls can be made of glass, and the best ones are covered with a layer of finely ground fish scales to give them luster.
While they might look like pearls, their surfaces will always be smoother and softer, and your sensitive front teeth can tell the difference.