A karat is a unit used to indicate purity in gold. 24-karat gold, for example, is pure gold. However, because pure gold is too soft for use, it is usually mixed with another type of metal such as copper or silver. If the metal used in a piece of jewelry is made of metal that is 18 parts gold and 6 parts copper, the metal is then 18-karat gold. A karat should not be confused with a carat, which is a unit of weight used to measure the size of a diamond or other gemstone.
