A pearl is a organic gem produced by saltwater or freshwater oysters or mollusks.  A small foreign object, such as a parasite, finds it’s way into the shell and into the tissue of the mollusk.  If this parasite or foreign body becomes trapped, the mollusk cannot get rid of the irritating intruder.  The mollusk then produces a blackish substance called concholin, over which another substance called nacre is then secreted. 

     
Nacre is the lustrous coating that a pearl is prized for, an opalescent material, a rainbow like glow of light and colors.  A pearl is the result of the build up of layers upon layers of the pearl nacre,  the thicker the nacre, the more beautiful the pearl.