Pearls One of the largest saltwater pearls still in existence is the Hope Pearl.. It is on display at the British Museum of Natural History.  It is two inches long, and varies between31/4 and 41/2 inches in circumference. A natural pearl forms when an irritant works its way into a particular species of oyster, mussel or clam. As a defense mechanism, the mollusk secretes a fluid to coat the irritant. Layer upon layer of this coating is deposited on the irritant until a lustrous pearl is formed.     A cultured pearl undergoes the same process. The only difference is that the irritant is a surgically-implanted mother-of-pearl bead or piece of shell. The core is, therefore, much larger than in a natural pearl. As long as there are enough layers of nacre to result in a beautiful, gem-quality pearl, the size of the nucleus is of no importance to beauty or durability. The Persian Gulf has always been the source of the finest natural saltwater pearls. Other sources are the waters around Sri Lanka, Australia, Japan, Mexico, Panama, Venezuela and the Micronesian Islands. Japan is the major source of cultured saltwater pearls, with Burma and Australia contributing to world supply. The most popular colors for round pearls are whites, creams and pinks. Silver, black and gold are gaining new interest. Freshwater pearls occur naturally, but in recent years a strong cultured pearl industry has sprung up for this product. Freshwater pearls are generally very irregular in shape, with a puffed rice appearance Pearls were once considered an exclusive privilege for royalty, for the making of their pearl and gold jewelry. Pearls are also available in the form of loose strands to use in the making of beautiful gold jewelry