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also karat, mid-15c., from Middle French carat "measure of the fineness of gold" (14c.), from Italian carato or Medieval Latin carratus, both from Arabic qirat "fruit of the carob tree," also "weight of 4 grains," from Greek keration "carob seed," also the name of a small weight of measure (one-third obol), literally "little horn" diminutive of keras "horn" (see kerato-).
Carob beans were a standard for weighing small quantities. As a measure of diamond weight, from 1570s in English. The Greek measure was the equivalent of the Roman siliqua, which was one-twentyfourth of a golden solidus of Constantine; hence karat took on a sense of "a proportion of one twentyfourth" and became a measure of gold purity (1550s). Eighteen carat gold is eighteen parts gold, six parts alloy. It is unlikely that the classical carat ever was a measure of weight for gold.
unit of weight for diamonds and certain other precious gems. Before 1913 the weight of a carat varied in different gem centres. Originally based on the weight of grains or leguminous seeds, which, of course, varied in size from place to place, the carat was equivalent to 0.2053 gram (3.168 troy grains) in London, 0.1972 g in Florence, and 0.2057 g in Amsterdam. The weight of a gemstone was calculated in terms of whole carats plus fractions (12, 14, 18, 116, 132, or 164) of a carat; thus, a stone might be said to weigh 3 + 14 + 116 carats. After various unsuccessful attempts to standardize the carat, the metric carat, equal to 0.200 g, and the point, equal to 0.01 carat, were adopted by the United States in 1913 and subsequently by most other countries. The weights of diamond, ruby, sapphire, emerald, topaz, aquamarine, garnet, tourmaline, zircon, spinel, and sometimes opal and pearl are expressed in carats