<Dd> M is the mass of pure gold in the alloy, and </Dd> <Dd> M is the total mass of the material . </Dd> <P> Therefore, 24 - karat gold is pure (while 100% purity is unattainable, this designation is permitted in commerce for 99.95% purity), 18 - karat gold is 18 parts gold, 6 parts another metal (forming an alloy with 75% gold), 12 - karat gold is 12 parts gold (12 parts another metal), and so forth . </P> <P> In England, the karat was divisible into four grains, and the grain was divisible into four quarts . For example, a gold alloy of ⁄ fineness (that is, 99.2% purity) could have been described as being 23 - karat, 3 - grain, 1 - quart gold . </P>

How does the karat system work for gold
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