Diamond carat is the most widely known criteria of the four's. Due to their similar names, carat is often confused with the measurement for gold purity - karat. One carat is equal to 200 milligrams. This measurement can be further divided into points, which is equal to a ten thousandth of a carat (0.02 mg). The name carat originates from the ancient Greek name for the carob tree (kertion, literally "small horn"), which was a popular method of counterweighting metals and stones.
Despite the term having originated many thousands of years ago, there was no standard weight throughout the world, and each country or empire had its own interpretation of the weight. The carat only became a standardised measurement in the early to mid 20th century, starting with the U.S. adopting the measurement in 1913. The U.K. and Europe soon followed suit, and by the 1930's, the entire industry and every major economy had agreed to use carat as a measurement.