The method by which fineness of gold is expressed. Pure unalloyed gold is 24 karat. As an alloy metal is added (usually copper), the karat value declines: 22K, 20K, 18K, and so on. The lowest grade of gold to carry a karat mark in the United States is 10K, or, in Great Britain, 9K. Most gold coins are 20K or 21K. Jewelry is commonly 9K to 18K. The word karat derives from the carob bean, used as a measure of weight in the ancient world. When spelled "carat", it refers to the weight of a precious gem and has nothing to do with the fineness of a metal in the United States.