Carat

Carat (ct.) is the basic measuring unit of diamonds - how much a diamond weighs. One carat equals .200 grams (or 200 milligrams). Diamonds are weighed to a thousandth (0.001) of a carat and then rounded to the nearest hundredth. Over a carat, diamond weights are usually expressed in carats and decimals. Some weights are considered "magic sizes" – half carat, three-quarter carat, carat.