carat

This term serves two distinct functions in the jewelry trade. When referring to gemstones, it is a strict unit of mass, where one unit equals exactly 200 milligrams. This allows for a standardized global market for diamonds and other precious stones. When applied to gold, the term describes a ratio of purity rather than weight. A 24 carat gold piece is chemically pure, while lower numbers indicate the proportion of gold relative to other alloys. It is important to distinguish this from karat, which is the common American spelling for gold purity, though carat is used globally for gemstones.