Popular myth would have you believe that a pearl forms when a grain of sand gets into an oyster's shell. This is hardly ever the case. Usually a pearl is created when an irritant such as a wayward food particle gets trapped in the mollusk. A pearl begins to form when this irritant is coated by the mollusk with the same substances the animal uses to create it's shell, aragonite and conchiolin, the combination of which is called nacre. After several years of the layers of the nacre building up, a pearl is formed.