CARAT, a unit of weight for diamonds, other gems, and pearls; it also indicates the purity or fineness of precious metals. Originally grains or leguminous seeds were used for the weighing of precious stones and as these were not of uniform size or weight the carat in different gem centres varied considerably. Thus, the London market for many years used a carat equal to 0.2053 grams (3.163 troy grains) ; in Florence it was 0.1972 grams; in Amsterdam 0.2057 grams, and so on. After various attempts to simplify and standardize the carat, the metric carat of 0.200 grams, or 200 milligrams, was adopted in the United States in 1913, and has now become the standard in the principal countries of the world. The metric carat permits the weight of gems to be expressed conveniently in decimals, whereas formerly it was customary to use a clumsy series of fractions, such as 1, 4-, , 116, 132, and 164 of a carat.
When the term carat is used to indicate the purity or fineness of metals it means 124th part. Thus, i8-carat gold consists of eighteen parts of gold and six parts of alloy. See Kraus and Holden, Gems and Gem Materials.