Pearls are formed by mollusks - a group of animals that includes clams, oysters, snails and squid. When an irritant, such as a bit of food, grain of sand, or even a piece of the mollusk's mantle becomes trapped inside the mollusk, it secretes substances to protect itself - aragonite (a mineral) and conchiolin (a protein). These substances are secreted in layers and a pearl is formed.
Depending on how the aragonite is arranged, the pearl may have a high luster (nacre, or mother-of-pearl) or a more porcelain-like surface.
Image: Are they real? Probably not. Pearls, Courtesy Milica Sekulic, Flickr
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