Carat

Carat (purity)

For other uses of the word "carat", see Carat.
The carat (abbreviation ct or K) is a measure of the purity of gold and platinum alloys. In the United States and Canada, the spelling karat is used, while the spelling carat is used to refer to the measure of mass for gemstones (see Carat (mass)). As a measure of purity, one carat is purity by mass:



where

X is the carat rating of the material,
Mg is the mass of pure gold or platinum in the material, and
Mm is the total mass of the material.
Therefore 24-carat gold is pure (100% Au w/w), 18-carat gold is 75% gold, 12-carat gold is 50% 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% 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The carat system is increasingly being complemented or superseded by the millesimal fineness system in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy.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, jewellery 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)