Principally there are two kinds of pearls, natural pearl and cultured pearl. `Natural Pearl are the result of misplaced nacre-producing mantle cells. The only cells that can produce nacre are those of the outermost skin of the mantle, which lies against the shell. These cells are probably captured inadvertently by some kind of fCultured Pearloreign body (such as a small stone, a piece of shell, grain of sand, or even a parasite) when penetrating the mantle tissue. The cells are able to reproduce themselves and surround the intruding object while the nacre continue to coat it, forming a pearl. Natural pearls are so rare that thousands of oysters might have to be opened in order to reveal a single pearl and even then the pearl is likely to be tiny, misshapen and practically worthless. Cultured Pearls are the fruit of a close co-operation between their mother oysters, Nature and Man.