Yellow Gold
Gold is graded by carats. The carat system for gold, unlike diamonds, is a method of expressing the proportion of gold to other metals in a particular alloy. Pure gold is 24ct. 22ct gold contains 22 parts of gold to 2 parts of other metals, while 9ct gold contains 9 parts of gold to 15 parts of other metals.
The colour of pure gold is a metallic yellow. However, copper and silver are the principal metals used for gold alloy, though zinc, cadmium, iron and aluminium are also used. The refiner's problem is to reach a satisfactory compromise between working qualities and colour for each different carat quality.