What Are the Marks for 18 Kt Gold?

Gold jewelry is often stamped with certain marks that provide information on its purity. These marks help the consumer to determine the value and authenticity of a piece of jewelry. Most jewelry has at least one mark designating its gold content. Consumers wanting to purchase 18 karat jewelry should look for one of three marks.

Karat refers to how much gold is in a particular item. Gold jewelry can range from 24K to 10K; 24K is 100 percent pure gold while 10K is 41.6 percent pure gold. A piece of jewelry stamped 18K means that it is 75 percent pure gold. While 24K is 100 percent pure gold, it is actually not the most desirable mark in jewelry, because pure gold is too soft. To increase durability and hardness, the gold needs to be mixed with other metals such as silver, nickel, copper, zinc and palladium. Other variations of 18K are 18 Karat, 18 Kt, 18 Karat Gold, 18 K. Gold and 18 Kt. Gold.

The 18K mark is used in the U.S. and Japan. In Europe, 18 karat gold is marked 750, in reference to it being 75 percent pure. Under the percentage system, 10 karat gold is marked 417, 14 karat is marked 583, 22 karat is marked 916, and 24 karat is marked 100. Both the karat and percentage marks are widely used in other parts of the world.

A piece marked 18K Plumb or 18KP still means that the piece is 75 percent pure gold. "P" or "Plumb" is simply an old-fashioned term that means that the piece has a gold content level exactly as it is stamped. In the U.S., this means that something marked 18K Plumb or 18KP must be within three parts per thousand of 18 karats. By marking the jewelry piece with "Plumb" or "P," the jewelry manufacturer is asserting that its piece complies with the law.

A piece marked 18K RGP or 18K Gold Overlay should not be confused with 18K. These marks mean that the piece has a layer of 18 karat gold bound to a base metal. This plating eventually wears away, depending on how thick the plating is and how the piece is worn.