Carat (purity) - Derivation

Derivation

"Karat" is a variant of "carat". First attested in English in the mid-15th century, the word "carat" came from Middle French carat, in turn from Italian carato, which came from Arabic qīrāṭ (قيراط), which came from Greek kerátion (κεράτιον) meaning carob seed (literally "small horn") (diminutive of κέρας - keras, "horn") and was a unit of weight though it was not likely used to measure gold in classical times.

In 309 CE, Roman Emperor Constantine I began to mint a new gold coin solidus that was 1⁄₇₂ of a libra (Roman pound) of gold equal to a mass of 24 siliqua, where each siliqua (or carat) was 1⁄₁₇₂₈ of a libra. This is believed to be the origin of the value of the karat.