Gold Purity and Colours

Gold in its pure form (24k) is too soft to withstand abrasions caused from extensive daily wear, so it is alloyed with other metals to give it strength. This strength makes gold an excellent choice for a ring setting. Common gold alloys are silver, copper, nickel, and zinc.

Gold colour is determined by the percentage of alloys that are included in the metal. When gold is alloyed with silver, copper, and zinc, the shade of colour will vary. When gold is alloyed with nickel, copper, and zinc, it becomes white gold. Yellow gold and white gold have very similar strength and malleability. White gold looks very similar to platinum, but the two have very different properties and prices. 
The purity of gold is measured in karats, which are expressed in 24ths. Thus, 24k gold is pure while 12k gold is 50 percent gold and 50 percent alloy.

The addition of alloying elements (other metals) to gold are used to increase the toughness and hardness of the metal, as well as change the colour. Additional metals enhance properties such as castability, grain size, hardness, corrosion resistance, colour, workability, ultimate strength, and others.

For example: 18k rose gold is 75%, or 18 parts fine gold and 25%, or 6 parts copper. It is the rich red copper combined with the pure yellow gold that creates a warm rosy tone. 14k white gold is 14 parts gold and 10 parts white metal, either nickel or palladium. These white metals dominate the colour, creating a warm grey tone.

Typical metals used to achieve the desired gold colour are:

Copper - Reddening
Silver - Greening
Zinc - Bleaching
Nickel - Whitening
Palladium - Whitening