The karat, abbreviated K or kt, is a unit used by jewelers in the United States to indicate the purity of gold alloys. It is not the same as the caret, a weight unit used for gemstones.
The karat designation of a gold item tells how much gold is present in the alloy used to make the item. Pure gold is 24-karat (24K), so a number smaller than 24 indicates how many 24ths of the alloy are gold. For example, 12K gold contains a minimum of 12/24 gold, or 50 percent.
Pure gold is too soft to use for jewelry. Handling and normal wear would quickly blur or remove any designs. Gold coins and jewelry are typically 22K gold, or 91.67 percent gold with other metals added to give the gold hardness. A gold coin such as the Krugerrand contains one ounce of pure gold plus a small added weight of other metals, and thus weighs more than one ounce.
Gold designated 10K contains 37.5 percent pure gold and 62.5 percent other metals. Metal of less than 10K gold content cannot be sold as "gold" in the U.S. A typical 10K gold alloy contains 37.5 percent gold, 52 percent silver, 4.9 percent copper, 4.2 percent zinc and 1.4 percent nickel. "Rose" 10K gold contains a higher percentage of copper and slightly more gold, lending it a reddish cast.