Pure gold is referred to as fine gold. Fine gold is not typically used in jewelry because it is very soft and can easily scratch. The market for pure gold jewelry is small; consumers usually consider pure gold pieces to be more of an investment rather than items to be worn.
Gold jewelry is usually alloyed with metals such as silver, copper, zinc, and nickel to reduce its cost and to change its characteristics. Gold alloys increase the hardness of the metal, which improves wear and permits a higher polish and brighter luster. Gold alloys also have a vivid range of color.
Alloying gold with copper produces a red or pink color; silver produces a greenish color; iron a blue color; bismuth a black color; aluminum a purple color; and nickel or palladium a white color.
Many of the white gold alloys were developed after World War II as substitutes for platinum. Consumers had developed a preference for platinum’s white sheen in their jewelry. At the time, however, platinum was considered a strategic metal, essential to the war effort and jewelry manufacturers were forbidden from using it.
Gold alloys usually weigh less than pure gold and they are also less expensive, making fine jewelry more affordable. Some people are sensitive to the metals used to alloy gold, but this allergic reaction can often be avoided or eliminated.
The gold content of an alloy is described in two ways: fineness and karatage . Fineness is a measure of purity expressed in parts per thousand. Pure gold is said to be 999 parts fine. Gold with a fineness of 750 has 750 parts (75%) gold and 250 parts of other metals.
The purity of gold is also expressed in a measurement called a karat (or carat). One karat is 1/24 pure, so 24 karat is pure gold. An item made of 18 karat gold is actually an alloy containing 18 parts gold and 6 parts of another metal or metals. Karat is abbreviated as “K” or “Kt.” Outside the U.S. “karat” is often spelled “carat,” but this should not be confused with the unit of weight for gemstones, also measured in “carats.”
The relationship of these two types of measures is summarized below.
Karat 24K 18K 14K 10K
% precious metal 99.9% 75% 58.5%* 41.7%
Parts per thousand 999.99 750 585* 417
* In the U.S., 14K is defined as 583 fineness.