In 1986, schoolchildren in western Kentucky petitioned their state representative to propose pearl as the state gemstone, and the Kentucky General Assembly made the designation official on July 15, 1986 (Kentucky Acts ch. 488, sec. 2). Freshwater pearls can be found in Kentucky and are attractive, but they are not typical gemstones. Gemstones are usually minerals. Pearls are deposits of calcium carbonate (aragonite, calcite, or both) called nacre. Pearl (nacre) is not a mineral because it does not have a distinctive crystal structure and because it is formed by the action of a living organism. Minerals are formed without the actions of living organisms. Because pearls have long been used in jewelry, however, they are considered gemstones.
Pearls form around irritants (usually sand grains) within a pelecypod shell (usually clam, oyster, or mussel). Freshwater pearls are the state gemstones of both Kentucky and Tennessee. Natural freshwater pearls were historically found throughout the Mississippi and Tennessee River valleys. Damming of these rivers, over-harvesting, and increased pollution severely depleted the population of natural pearl-producing mussels in these rivers. Today, however, mussels with freshwater pearls are cultivated through man-made processes on "pearl farms" along Kentucky Lake in Tennessee. Cultivation just across the border from Kentucky is a multi-million dollar industry, providing pearls to the Japanese cultured pearl industry. Numerous species of freshwater mussels (Unionidae), especially Megalonaias nervosa (washboard mussels) and Fusconaia ebena (ebony mussels), are used for cultivating commercial freshwater pearls. Different species produce different colors and shapes. It takes 2 to 3 years to cultivate a pearl. Not only are the pearls themselves harvested, but mussel shells are used to produce round "beads" that are sold abroad to use as the irritants around which cultivated pearls in "pearl farms" form.