Gold purity is measured in "karats", and pure,100% gold is 24K gold. 24K gold is soft and malleable for a metal, so jewelers add alloys to the gold and strengthen the metal. This helps ensure the gold watch doesn't scratch easily and lose mass.
The most common alloys are copper and silver. Because watches are so susceptible to scratching jewelers rarely use more than 18K gold for gold watches.
Below are composition percentages for a yellow gold watch:
14K yellow gold watch composition
18K yellow gold watch composition
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