Francis H. Brown, dean of the College of Mines and Earth Sciences at the University of Utah, provides the following answer.
Meanings of the word "gold" are manifold. The term is used to refer to the chemical element itself (its symbol is Au) and also to objects (often jewelry) that contain some gold but do not consist entirely of gold¿a gold watch or ring, for instance. In the latter case, the amount of gold is described by the karat fineness of the alloy, so one sees marks such as 12K for 12 karats, and so forth.
The purity of gold is defined either in karats or fineness. A karat is 1/24 part of pure gold by weight, so 24-karat gold is pure gold. To find the percentage of gold in an object when the purity is stated in karats, multiply the number of karats by 100 and divide by 24. For example, 12-karat gold is 50 percent pure gold. Alternatively, the fineness is the amount of gold in parts per 1,000, so pure gold has a fineness of 1,000. A gold object containing 800 fine gold, or a fineness of 800, is 80 percent gold.
Alloys are mixtures of two or more metals, and many metals form alloys with gold. Most gold alloys are mixtures of gold, copper and silver. Gold alloyed with copper takes on a reddish color and is commonly 18K or less. If some of the copper is replaced with silver (keeping the gold content constant), the alloy takes on a yellow green hue or a rose color, depending on the mixture of copper and silver. So-called green or very pale yellow gold has been alloyed with silver but no copper. White gold used to have additions of nickel, copper and silver, but because of allergies, palladium is now used in place of nickel. Zinc is also used in gold alloys