Etymology Dictionary

CARAT,  a certain very light weight.  (F.,—Arab.,—Gk.)   Generally a weight of 4 grains.   In Shak. Com. Err. iv. 1. 28.—F. carat, 'a carrat; among goldsmiths and mintmen, is the third part of an ounce, among jewellers or stone-cutters, but the 19 part;' Cot.   Cf. O. Port. quirate, a small weight, a carat; cited by Diez.—Arab. qirrát, a carat, the 24th part of an ounce, 4 barley-corns; also, a bean or pea-shell, a pod, husk; Richardson's Arab. Dict. p. 1122.—Gk. κεράτιον, the fruit of the locust-tree; also (like Lat. siliqua), a weight, the carat; the lit. sense being 'a little horn.'—Gk. κέρας (stem κερατ-), a horn, cognate with E. Horn, q.v.   ¶ The locust-tree, carob-tree, or St. John's-bread-tree is the Ceratonia siliqua; 'The seeds, which are nearly of the weight of a carat, have been thought to have been the origin of that ancient money-weight;' Engl. Cycl. div. Nat. Hist. s.v. Ceratonia.   There need be little doubt of this; observe further that the name Cerat-onia preserves the two former syllables of the Gk. κεράτ-ιον.   See Carob, which is, however, unrelated.