also karat, late 15c., "a measure of the fineness of gold," 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, literally "little horn" diminutive of keras "horn of an animal" (from PIE root *ker- (1) "horn; head").
Carob beans were a standard in the ancient world for weighing small quantities. The Greek measure was the equivalent of the Roman siliqua, which was one-twenty-fourth of a golden solidus of Constantine; hence karat took on a sense of "a proportion of one twenty-fourth, a twenty-fourth part," especially in expressing the fineness of gold when used as jewelry, and thus is became a measure of gold purity (1550s): 18-carat gold is eighteen parts gold, six parts alloy; 14-carat gold is 10/24ths alloy, etc.
As a measure of weight for diamonds or other precious stones, carat is attested from 1570s in English. In U.S., karat is used for "proportion of fine gold in an alloy" and carat for "measure of weight of a precious stone."