The carat (ct) is a unit of mass equal to 200 mg (0.00705 oz) or 0.00643 troy oz, and is used for measuring gemstones and pearls. It's also known as metric carat.The carat is divisible into 100 points of 2 mg. Other subdivisions, and slightly different mass values, have been used in the past in different locations. In terms of diamonds, a paragon is a flawless stone of at least 100 carats (20 g).
Carat comes from the Carato Italian word. Carob seeds have been used throughout history to measure jewelry, because it was believed that there was little variance in their mass distribution. However, this was a factual inaccuracy, as their mass varies about as much as seeds of other species.