: one of the rounded concentric masses of squamous epithelial cells characteristic of certain tumors
: a miliary leproma of the iris
: a rounded abnormal mass of enamel on a tooth
Blister pearl attached to the shell of an Oriental pearl oyster (Pinctada martensii Dunker)—Courtesy of The American Museum of Natural History, New York
Concretion formed by a mollusk and consisting of the same material (called nacre, or mother-of-pearl) as the mollusk's shell. Long treasured as gemstones, pearls are valued for their translucence and lustre and for the delicate play of surface colour. The more perfect a pearl's shape and the deeper its lustre, the greater its value. The colour varies with the mollusk and its environment. Jewelers of the 16th–17th centuries often used irregularly shaped (baroque) pearls, formed from muscular tissue, to form the bodies of animals and other figures. In Europe and China, mother-of-pearl has been used as an inlay material for decorating furniture. The discovery that a pearl could be cultivated by insertion of a foreign object inside the mollusk's shell is said to have been made in 13th-century China.