Gold jewelry has been popular for thousands of years, and standards for its measure and purity have evolved over time. Before investing in gold jewelry, it's helpful to understand the various types of gold jewelry and its stamps and marks.

Gold has been traded throughout recorded history. Pure gold has the highest value, but it is rarely used in jewelry, as it is too soft to allow daily wear. To overcome this issue, gold is mixed with stronger metals, which decreases its value. To determine the difference in value between pure gold and gold mixed with other metals, various systems were developed to identify the purity of gold.

In the United States, 100 percent pure gold is marked "24 karat." Gold with lower gold content is marked according to its percentage: 18-karat gold is 75 percent pure; 10-karat is 42 percent pure. Europeans mark their gold with a decimal percentage system: Pure gold is marked "1,000," 18-karat gold is marked "750," and so on.

A piece of jewelry that bears one of the following marks is not gold: G.E.: gold electroplate H.G.E.: heavy gold electroplate G.F.: gold filled 925: sterling silver If it appears to be gold but is marked 925, it is probably what is known as vermeil, a thin layer of gold coating a solid base of silver. Vermeil can be beautiful, but it has little long-term value.

Unmarked gold-colored jewelry is most likely simply costume jewelry of little value, but there are a few exceptions to this rule. Jewelry that was made before 1900 may not have a gold mark, and jewelry that was made by an artist or independent jewelry designer may not be stamped. The mark on a piece of jewelry that has had daily wear for many years may be worn away, and a ring that has been sized many times may have had its mark cut out or smoothed over.

If you cannot find a mark on a piece of jewelry, have it tested by a reputable jeweler. If you are considering buying a piece of unmarked jewelry on eBay or a similar auction site, it's best to assume the piece is a fake. Most auction sellers know exactly what their jewelry is worth, and if a seller believed a piece was gold, he would have tested it to make sure he received top dollar for it.