A carat is a unit of measurement representing the weight of precious stones, pearls, and certain metals (such as gold). It was originally a unit of mass based on the carob seed or bean used by ancient merchants in the Middle East. In terms of weight measurement, a carat equals three and one-fifth grains troy, and it is also divided into four grains (sometimes referred to as carat grains). Diamonds and other precious stones are estimated by carats and fractions of carats; pearls are usually measured by carat grains (for more about grains and measurement, see “Mathematics throughout History”).

Carats of gold are measured based on the number of twenty-fourths of pure gold. For example, 24-carat gold is pure gold (but for a goldsmith’s standard, it is actually 22 parts gold, 1 part copper, and 1 part silver, as real gold is too malleable to hold its shape), 18-carat gold is 75 percent pure, 14-carat gold is 58.33 percent pure, and 10-carat gold is 41.67 percent pure gold.