Karat denotes the purity of the metal. 24 karat is pure. So 18 karat, for example, is 18 parts gold and 6 parts other metals. This is reflected by a hallmark of "18K" or "750" (18/24ths). Carat is the weight (not the diameter) of a diamond or gem. Carat is a metric weight = 1/5th of a gram. The term is derived from the carob seed which in ancient times was used as a universal weight measure due to its consistency in weight regardless of where grown in the world.