Pearl is included among gemstones only because it is a beautiful object used as jewelry. As has been noted, pearl is not mineral because it is formed by the action of a living organism. However, the pearl has long occupied an important position among jewels, and it is unique in requiring no lapidary art to enhance its beauty. Nature has perfected pearls. [Illustration: The strand of matched pearls was presented to President Van Buren by the Imam of Muscat. The three baroque (irregularly shaped) pearls are freshwater pearls from the Wabash River in Indiana.] The ancient Chinese believed that pearls originated in the brain of a dragon. We now know, of course, that pearl is created by a secretion of a mollusk. Very few mollusks have the ability to produce the fine mother-of-pearl used in the jewelry trade, and even among those that can, very few produce pearls with iridescence, or _orient_, as it is known in the trade. Only two genera, the pearl oyster (_Margaritifera_) and the pearl mussel (_Unio_) are important sources of the gem. Edible oysters rarely produce pearls, and when they do, the pearls are of poor quality. The shells of pearl-producing mollusks are composed of layers of calcium carbonate in the form of either calcite or aragonite. These layers, cemented together with an organic substance known as conchiolin, are known as nacre. The layer closest to the animal is deposited in tiny overlapping patches, producing an iridescent effect caused by the interference of light rays reflected from the plates making up the nacre. The same material coats the surface of a gem pearl. Seldom does a mollusk live out its time without attack by creatures boring through its shell, or without intrusion through the normal shell opening of tiny parasitic worms, sand, or other irritants. Usually inert particles are forced against the inside of the shell, where they are covered with layers of pearl that fasten them to the shell. This is the source of most _blister pearls_. When the irritant remains in its fleshy part, the mollusk deposits a protective shell of pearl to cover it completely, and a spherical pearl may result. Pearls of less-symmetrical shape, called _baroques_, are more common.