These are formed naturally. When a foreign particle enters the shell of a mollusk, its presence irritates the inner surface of the shell of the mollusk. Thus, in order to get rid of this irritation, the mollusk produces a chemical substance, calcium carbonate and conchiolin, to cover this foreign particle with. The gradual secretion and cementing of the chemicals on the particle, results in the formation of a pearl. It can take any shape and perfect round shapes are seldom seen.