The main difference between natural and cultured pearls is whether or not humans intervened in the process. Unlike cultured pearls, natural pearls are made by chance. A tiny organism enters the mantle of a mollusk, and as a defence mechanism, the mollusk secretes multiple layers of nacre to cover it over a long period of time. It is extremely rare nowadays to find natural pearls. In order to meet the demand, cultured pearl farming is practiced in many parts of the world.