carat
n. countablen. a unit used to measure how heavy a diamond or other gemstone is. It is also used to describe how pure gold is.
n. a unit of weight for precious stones, equal to 200 milligrams; also a measure of the purity of gold, where 24 parts represent pure metal.
She wears a two-carat diamond ring.
The jeweler explained that a higher carat rating for gold means the metal is softer but more valuable.
While the size of the gemstone is impressive, its value is determined by the combination of its carat weight, clarity, and the precision of its cut.
From Middle French carat, from Italian carato, from Arabic قِيرَاط (qīrāṭ, “carat, similarly small units such as inches”), from Ancient Greek κεράτιον (kerátion, “hornlet, carob seed”), from κέρας (kéras, “horn”) + -ιον (-ion, diminutive suffix). Doublet of quilate and keratin.
In the context of gold purity, often spelled 'karat' in American English.