Gem, mineral or stone prized for its beauty and rarity and durable enough to be used in jewelry and for ornaments. Most types of gem are found in igneous rocks. The chief ones have a hardness of 8 or more on the Mohs' scale and are relatively resistant to cleavage and fracture, though some are fragile. They are identified and characterized by their specific gravity and optical properties, especially the refractive index. Gems of high refractive index show great brilliancy and prismatic dispersion (“fire”). Other attractive optical effects include chatoyancy (changeable color or luster), dichroism (different color in different light), opalescence (reflection or iridescence), and asterism (a star-shaped gleam caused by regular intrusions in the crystal lattice). Since earliest times gems have been engraved. Somewhat later cutting and polishing were developed, the cabochon (rounded) cut generally being used. In the late Middle Ages faceting, now the commonest cutting style, arose. The most valuable gems are diamonds, rubies, sapphires, and emeralds. Other materials—such as aquamarine, garnet, jade, opal, turquoise—are considered semiprecious stones. Amber, coral, and pearls are gems of organic origin.