Word Origin & History
1469, from M.Fr. carat, from It. carato, from Arabic qirat "pod, husk, weight of 4 grains," from Gk. keration "carob seed," lit. "little horn" dim. of keras "horn." Carob beans were a standard for weighing small quantities. As a measure of diamond weight, from 1575. The Gk. measure was the equivalent
of the Roman siliqua, which was one-twentyfourth of a golden solidus of Constantine; hence the word took on a sense of "a proportion of one twentyfourth" and became a measure of gold purity (1555). Eighteen carat gold is eighteen parts gold, six parts alloy. It is unlikely that the carat was ever a measure of weight for gold.