Keshi pearls, also called 'chance' pearls, are usually baroque shaped, accidentally produced in saltwater oysters used for cultured pearls production. Sometimes an oyster rejects its bead implant, but particles of the accompanying mantle tissue used alongside the bead remain; these particles of mantle tissue stimulate the production of nacre, resulting in the wonderful, interesting pearls we know as 'Keshi'. They are unusual because, like natural peals, they are essentially all nacre, and all natural.