Other forms: carats
A carat is the unit by which gems are weighed. A two-carat diamond, carat weight, half a carat.
One carat is exactly two hundred milligrams, standardised internationally in 1907 — before that it varied by market. It measures weight and not size, so two stones of the same carat can look quite different. The word is also used, spelt karat in America, for the purity of gold in twenty-fourths: 18 carat gold is eighteen parts in twenty-four. Note the name comes from the carob seed.
1nouncaratThe unit by which gems are weighed.
“He bought her a diamond ring of just under one carat.”
2nounmeasureA measure of gold purity in twenty-fourths.
“Nine carat gold is roughly a third pure.”
carat is a unit of measure.
Arabic qirat comes from Greek keration, a little horn, meaning a carob seed. Carob seeds were used as small weights, and their reputation for uniformity is largely a myth.
Ancient Greek
κεράτιον — hornlet, carob seed
Arabic
قِيرَاط — carat, similarly small units such as inches
Italian
carato
Middle French
carat
English, today
carat
Built from
From Middle French carat, from Italian carato, from Arabic قِيرَاط (“carat, similarly small units such as inches”), from Ancient Greek κεράτιον (“hornlet, carob seed”), from κέρας (“horn”) + -ιον. a doublet of quilate and keratin.
Etymology from Wiktionary, CC BY-SA