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For centuries, humans have treasured pearls. The lustrous play of light across the surfaces of good pearls is so attractive that people have paid fortunes for them, even though they have no human use except adornment. Pearls actually come in many colors, sizes, and shapes, and are ranked in value according to these qualities. Perfectly round ones with a deep glowing luster, particularly in unusual colors that also show an iridescence (or orient ), have always been the most prized and expensive; dull, irregular ones the least. Pearls come from a group of water organisms called pearl molluscs, which includes oysters, mussels, and clams from both freshwater and saltwater. The pearl itself actually begins as an irritant. Sand, a pebble, or a pesky parasitic organism gets inside the oyster's shell. To reduce the irritation, the oyster coats the intruder with layers of a solid, slick material called nacre. The oyster's mantle tissue secretes the two main components of nacre: thin layers of the mineral aragonite and a glue-like substance called conchiolin, which cements the layers together. Because the aragonite is translucent, light interacts with the overlapping layers to give the finished pearl its lustrous appearance. Pearl molluscs also coat the inside of their own shells with nacre, so some shells picked up at a seashore are shiny and iridescent inside. Pearls used to be harvested by divers. However, it is a dangerous occupation and natural pearls of high quality are rare. People have now learned to farm pearl molluscs specifically to produce cultured pearls, small beads with layers of pearl material around them. Oyster larvae (called spat ) are allowed to settle in sheltered locations underwater. Once they have attached themselves to ropes or rafts, the young oysters are grown for a few years. Then their shells are opened just wide enough to surgically insert a small pearl bead and a piece of mantle tissue from another mollusc into the soft tissue. This nucleation process provides the oyster with a spherical irritant to coat with nacre, increasing the likelihood of a symmetrical, round pearl. The farmer removes the cultured pearl from the oyster one to three years later. Cultured pearls, produced around the world, account for about 90 percent of all pearl sales.