Carats, abbreviated ct. and spelled with a c, is a measure of weight used for gemstones. The carat weight was standardized in the 20th century. The United States officially adopted the metric carat in 1913. One carat = .2 grams or 200 milligrams, equal to 1/5 of a gram. Stones are measured to the nearest hundredth of a carat. A hundreth of a carat is also called a point. Thus a .10 carat stone can be called either 10 points, or 1/10 of a carat.