Karat is the variant of carat. The word was originally a unit of mass (weight) based on the Carob seed or bean used by ancient merchants in the Middle East. The carob seed is from the carob or locust bean tree. The Romans also used the name Siliqua Graeca (Keration in Greek, Qirat in Arabic) for the bean of the Carob tree. The Romans also used the name Siliqua for a small silver coin which was one-twenty fourth of the golden solidus of Constantine. The latter had a mass of about 4.54 grams, so the Siliqua was approximately equivalent in value to the mass of 1 Keration or Siliqua Graeca of gold, i.e. the value of 1/24th of a solidus is about 1 Keration of gold, i.e. 1 carat.

Today carat is the unit of weight for gemstones where one carat equals 200 milligrams (0.200 grams), hence 1 gram equals to 5 carats. Karat is the unit of gold purity. It is a measurement indicating the proportion of gold in an alloy out of 24 parts, so 18K gold is 18/24 parts gold, and 24K is considered pure gold.