Therefore 24-carat gold is fine (99.9 per cent Au w/w), 18-carat gold is 75 per cent gold, 12-carat gold is 50 per cent gold, and so forth.
Historically, in England the carat was divisible into four grains, and the grain was divisible into four quarts.
For example, a gold alloy of fineness (that is, 99.2 per cent purity) could have been described as being 23-carat, 3-grain, 1-quart gold.The carat system is increasingly being complemented or superseded by the millesimal fineness system in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy.
The most common carats used for gold in bullion, jewell
ery making and by goldsmiths are:24 carat (millesimal fineness 999)
22 carat (millesimal fineness 916)20 carat (millesimal fineness 833)18 carat (millesimal fineness 750)15 carat (millesimal fineness 625)14 carat (millesimal fineness 585)10 carat (millesimal fineness 417)9 carat (millesimal fineness 375)