The pearl is the only gem produced within a living organism (a mollusk), specifically a saltwater oyster or freshwater mussel. The process begins when an irritant becomes lodged inside the organism. A nucleus then forms over the irritant and is coated over with many layers of nacre; this coating takes from 8 months to 6 years to complete. The size and shape of the nucleus along with the water in which the mollusk lives all affect the size, color and shape of the pearl.
Natural pearls disappeared from the mainstream jewelry market during the 20th century. This was due to the effects of pollution from oil drilling in the Persian Gulf, unchecked harvesting in the Australian pearl beds, over-fishing and destructive harvest methods on the North American continent. Current methods of culturing pearls have now made them an affordable luxury to the modern consumer.