The naturally occurring alloy of gold and silver is commonly known as electrum. Its colour depends on the ratio of gold and silver in the mix: a whitish alloy appears below 50 per cent gold, turning to greenish yellow as the amount of gold increases, and bright yellow at around 85 per cent gold. To get a wider range of colours, metallurgists and jewellers may add in a third metal. Introducing copper, for example, leads to reddy yellows and deeper brassy reds.
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