A pearl is formed when a foreign object gets into the shell of oyster or mollusk and becomes embedded in the tissue. In response, oyster or mollusk builds up layers of nacre surrounding the irritant and gradually forms a pearl.
Natural pearls are formed in nature by chance - a parasite or a piece of organic object accidentally embedded in the gonad or mantle tissue of a oyster or mollusk. Cultured pearls are formed by human intervention - inserting a foreign object into the tissue of an oyster or mollusk. But the process of pearl creation is same for both natural pearls and cultured pearls. The only difference is that natural pearls is created accidentally; but cultured pearl is created intentionally.