Most naturally occurring gold is impure. It usually contains small but significant amounts of other metals such as copper, silver, palladium and mercury.

These each give the gold distinctive colours; for example, gold that contains a significant amount of copper will be tinged red, while gold containing silver will be much paler than the distinctive and unmistakable bright-yellow of pure gold.

The purity of gold is measured in carats. 100% pure gold is defined as 24 carat. 18-carat gold is therefore 18/24, or 75% pure, while 14-carat gold would be 58% pure. Only certain carat values are recognized - 24, 22, 18, 14 and 9.

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