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Pearl, unlike gemstones or precious metals, is a kind of lively gem. A natural pearl forms when an irritant, such as parasite or a piece of sand, accidentally enters the body of a particular species of oyster, mussel or clam and cannot be expelled. As a defense mechanism, the mollusk secretes a smooth, crystalline fluid, called “nacre” (the same secretion it uses for shell-building, composed mainly of carbonated calcium), to coat the intruder. As long as the irritant is present, the mollusk continues to add a layer upon a layer of nacre on the irritant until a lustrous pearl is formed.