Carat, a unit of weight for precious stones. The international, or metric, carat was standardized in 1913 and accepted by the United States and many other countries. One carat is equal to 200 milligrams. A carat grain is 1/4 of a carat and is used in expressing the weight of pearls.
The term carat should not be confused with the term karat, which is not a unit of weight, but is used to indicate fineness or purity of gold. A karat is a 24th part. Thus, a gold ring of 18 karats contains 18 parts pure gold and 6 parts base metal by weight. A ring 24 karats fine is pure gold.