A pearl is a unique gem, the only one created by a living creature. Below is a detailed look at pearls, covering their formation, types, and characteristics. Unlike most other gemstones, which are minerals formed underground, pearls are **organic** gems. They are created inside the shells of certain mollusks Natural Pearls **Formation:** A natural pearl begins when an irritant—such as a piece of sand, a parasite, or other organic matter—becomes trapped inside a mollusk. To protect itself, the mollusk begins coating the irritant with layers of **nacre** (nay-ker), also known as "mother-of-pearl." Nacre is the same iridescent material that lines the inside of the mollusk's shell. The amount and quality of light that reflects off the surface of the pearl. High luster means the pearl looks bright, with sharp, clear reflections. This is often considered the most important factor. The number of blemishes, spots, cracks, or wrinkles on the pearl's surface. Fewer imperfections make the pearl more valuable. Perfectly round pearls are the rarest and most expensive. Other shapes include oval, button, drop, and baroque. Pearls naturally come in many colors (body color) and overtones (subtle hues of other colors visible when light hits the pearl).