A carat is a measure of gemstone weight (not to be confused with the karat which is a measure of gold purity or carrots, the long, thin, orange vegetable.)
A carat is 1/5 of a gram and was original used in ancient times as a convenient standard of weight because it referred to the weight of a carob bean before accurate scales had been developed. Carats are usually expressed in hundredths, ie. 1.00 (one carat) or 0.50 (1/2 carat.)
Remember that carat weight and gemstone size are not always equivalent in that gemstone size is also a function of the stones proportions as well as weight.
To Learn more about Diamond Carat, view GIA’s 4C Guide here.