What is a Carat?

A unit of weight for gemstones. Although it has varied in the past, the now generally accepted standard is the metric carat, one-fifth of a gram (200 milligrams), which was adopted in the U.S. on July 1st, 1913, and by Great Britain on April 1st, 1914.
The metric system divides the carat into 100 parts, resulting in such decimal weights as 0.24 ct., 1.35 cts, and the like.

Before acceptance of the metric carat, gem dealers had divided the carat into 64 parts and wrestled with such time-consuming fractions as 13/64 ct. or 1 44/64 cts.

The word carat comes from the seed of the carob tree, the biblical locust (Ceratonia siliqua), which was used by ancient pearl and gem dealers as a unit of weight.
The word is not to be confused with Karat, which denotes the ratio of fine gold and alloy in manufacturing fine jewelry.

A Portable Vintage Gem Scale