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How are karats used to show the gold content of jewelry?
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gold processing
The Tanami gold mine, Northern Territory, Australia.
In the context of gold processing, the karat system is used to indicate the fineness or purity of gold within an alloy.
A karat represents 1/24th part of the total mass of the object. Because pure gold is often too soft for practical use in jewelry, it is mixed with other metals to increase strength and reduce costs. The karat rating expresses how many of those 24 parts are gold.123
- 24-karat gold: Represents pure gold (24/24 parts).12
- 18-karat gold: Contains 18 parts gold and 6 parts other metals, resulting in a composition that is 75 percent gold by weight.23
Each karat is equivalent to approximately 4.1667 percent of the total content. While the karat system is standard for jewelry, the fineness of gold can also be expressed in parts per thousand, where 99.9 percent purity is referred to as "three-nines fine".13