What is the purity of gold defined in?

Gold purity is defined in both millesimal fineness and karats. Millesimal fineness is a system of expressing the purity of gold, silver, and platinum alloys in parts per thousand by mass. For example, an alloy labeled 999 means that 999 out of 1000 parts are pure gold.

 

Similarly, karats measure the proportion of gold to other metals in an alloy, with 24K being the purest form, which is equivalent to .9999 fine gold.

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