A unit of weight measurement for precious stones. It is important not to confuse this with the other karat (American spelling) or carat, which is a measure of the purity of gold. 'Carat' is abbreviated to "ct." One carat is equal to 1/5 of a gram (200 milligrams). Stones are measured to the nearest hundredth of a carat. There are a hundred points in a carat, so that a.10 carat stone can also be described as a 10 point stone. Smaller stones are most often referred to by point designations.

An average one-carat round diamond usually measures approximately 6.5mm in diameter. This relationship of weight and size, however, is different for each type of cut and differs for other gems. Rubies and sapphires, for example, are both heavier than diamonds, so a one carat ruby or sapphire is smaller in size than a one carat diamond.