The carat weight measures the mass of a diamond. One carat is defined as a fifth of a gram, or exactly 200 milligrams. The point unit – equal to one one-hundredth of a carat (0.01 carat, or 2 mg) – is commonly used for diamonds of less than one carat.

Larger stones are often more highly valued, but size should not be the only consideration–high brilliance, which varies according to clarity, cut, and colour grade, is highly desirable in a diamond.