Carat is the measurement unit used when weighing diamonds and gemstones. One carat is equal to 200 milligrams or 0.2 grams. It is important to note that carat weight is not the only determining factor when judging a gemstone’s size. Two gems with the same carat weight may be different sizes. The diameter for round gemstones, and length and width for other shaped gemstones is also considered when measuring a gem’s size.