Gold is naturally yellow in colour and will not tarnish, corrode or rust and is the Stones-Img-3most malleable of all metals. Pure gold is defined as 24 karat, and is too soft for most jewellery uses, so it is alloyed (mixed) with other metals to alter its properties, strength and colour.
The purity of gold is measured in karats, which are expressed in 24ths. Therefore, 24-karat gold is pure while 12-karat gold 50% gold and 50% alloy

• 24 Karat: This is pure gold and is too soft for jewellery

• 22 Karat: 91.7% gold, soft but is used for fine jewellery

• 18 Karat: 75% gold and most commonly used for fine jewellery

• 14 Karat: 58.3% gold and used for good quality jewellery

• 12 Karat: 50% gold and used for jewellery of a fair quality

• 9 Karat: 37.5% gold, most commonly used metal in UK.

White gold is an alloy of gold and some white metals such as nickel, silver and palladium. Yellow gold and white gold are very similar and tend to have the same alloy percentages. The only difference is the other metals use

An example:

18-karat yellow gold and 18-karat white gold are both made from 75% gold and 25% other metals. The difference between the two metals lies only in the alloy. The yellow gold is alloyed with coloured metals such as copper and zinc. The white gold is alloyed with white metals such as nickel and silver.
NB– Do not confuse karat with carat; carat refers to the weight of a diamond, while karat means the purity of the gold.