The US Federal Trade Commission grades gold to protect against fraud and guarantee the purity of metal used for jewelry. 24K gold is defined as pure gold. It is generally too soft for practical use as jewelry. While it can be hardened somewhat by the process of annealing (a process of heating and slow cooling), adding other metals to it to make alloys makes it more durable and lowers its cost.
24K is pure gold; occasionally marked ‘pg’. A karat is expressed in units of 24ths.
18K is 18/24, or 75% gold; the other 6 parts, or 25% consists of one or more metals.
14K is 14/24, or 58.5% gold, with 10 parts other metal(s).
12K is 12/24, or 50% gold, with 12 parts other metal(s).
10K is 10/24, or 41.7% gold, with 14 parts other metal(s). Less than a 10K designation cannot be called gold in the US.
Lower karat gold is more brittle and subject to discoloration and tarnish.
The addition of other metals also allows metallurgists to vary the color of gold.
There are a variety of patented and nonpatented alloys made by manufacturers.
White gold became popular as a substitute for the more expensive platinum around 1920. White gold is most commonly made by alloying it with nickel. A typical alloy might consist of 75% gold, 14.5% nickel, and smaller amounts of metals like copper and zinc, which increase hardness. However, since nickel can cause dermatitis in some people, palladium is sometimes used instead, although it is more difficult to process, more expensive and heavier. At the same time, palladium is softer and less durable. Other nickel-free alloys can be made with manganese, platinum, tin, silver, chromium, and iron; the color of these alloys is less white; they are more commonly made in the European Union (note that a rhodium plating is commonly added to provide the actual white finish; this is legal in many countries).
Rose and pink gold is made by adding copper. The more copper, the darker the color, and the stronger the alloy.
Green gold is made by adding silver. This slightly increases strength and hardness.