Carat, term expressing the ratio of precious metal to base metal in an alloy; also a unit of weight for precious stones. In the first sense, a carat (usually spelled karat) indicates 1/24 part by weight of a precious metal, such as gold, in an alloy. Thus, 18-karat gold is 18/24 or 3/4 gold, and 24-karat gold is pure gold. As a unit of weight for precious stones, the international metric carat, now used by most countries, was standardized by the U.S. government in 1913 at 200 mg, or 0.2 g. This standard carat is divided decimally; 0.01 carat is usually called a point.