The qualities sought in gems are beauty, rarity, and durability. The beauty of a gem depends primarily on its optical properties, which impart its luster, fire, and color; the durability depends on hardness and resistance to cleavage or fracture. The physical properties by which gems are distinguished from each other are form of the crystal, index of refraction of light, hardness, presence or absence of cleavage, type of fracture (conchoidal, even, or uneven) in stones without cleavage, specific gravity, color, streak (color of the powder as determined by rubbing it over white, unglazed porcelain), luster (appearance of the surface in reflected light—adamantine, vitreous, resinous, greasy, silky, or pearly), and transparency. Minor properties that serve to identify some stones are chatoyancy (changeable luster or color under undulating light), opalescence, asterism (starlike sparkling), play of color, fluorescence, phosphorescence, iridescence, and electrical properties. The unit of weight used for gemstones is the metric carat; one carat equals 200 mg.