A carat is a unit of measurement for the weight of gemstones, and 1 carat is equal to 200 milligrams. Since density can vary among gemstones, the lengths and widths of gemstones of equal weight varies.
The average approximate size of a 1-carat round diamond ranges from 6.2 to 6.8 millimeters in diameter. The term "carat" is derived from the word "carob bean." Carob beans were once used to balance scales to weigh gemstones because of their uniform weight. The carat was accepted as a standard unit of weight in 1907 by the Fourth General Conference on Weights and Measurements in Sevres, near Paris.