The basic unit is a carat, which is set at 200 milligrams (0.00704 ounces or 1/142 of an avoirdupois ounce). A well-cut, round diamond of one carat measures almost exactly 0.25 inch (6.3 millimeters) in diameter. Another unit commonly used is the point, which is one hundredth of a carat. A stone of one carat weighs 100 points. “Carat” as a unit of weight should not be confused with the term “karat” used to indicate purity of the gold into which gems are mounted.