One of the 4 “C”‘s of diamond grading, carat is a weight measurement. Diamonds, like other gemstones, are measured in metric carats. Each carat (not to be confused with karat, which refers to gold purity) equals .2 grams. It takes 5 carats to make a gram. Carats are measured in points. 100 points makes a single carat.
People think of carat as the size of the diamond. If you are comparing two diamonds, it’s ok to think of them as two different sizes. However two different types of gemstones with the same measurements may not be the same weight. See “Carat” in our glossary for more information.