Most jewelry is made from precious stones, mined from the earth.  But pearls are formed inside the shells of creatures known as pearl oysters.  Pearls are made of the same mother-of-pearl materials that lines the inside of the oyster’s shell.

They are formed when a grain of sand or other foreign object gets inside the oyster’s shell and irritates the oyster’s tender body.  To stop the irritation, the oyster coats the invader with layer after layer of mother-of-pearl.  After several years, a smooth, satiny ball has formed around the grain of sand, forming a lustrous pearl. – Dick Rogers