Carat (purity)

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For other uses of the word "carat", see Carat.

Carat or karat (abbreviation: ct or kt) is a measure of the purity of gold and platinum alloys. In the United States and Canada, the spelling karat is usually used for the measure of purity, while carat refers to the measure of mass (see Carat). As a measure of purity, one carat is one twenty-fourth purity by weight:

$X = 24 \frac{M_g}{M_m}$

where

X is the carat rating of the material,

M g

is the mass of pure gold or platinum in the material, and

M m

is the total mass of the material.

Therefore 24-carat gold is pure gold (99.99%), 12-carat gold is 50% purity, 18-carat gold is 75% purity etc.

The carat system is increasingly being complemented or superseded by the millesimal fineness system in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy.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 goldsmithing are:

24 carat (millesimal fineness 999)
22 carat (millesimal fineness 916)
20 carat (millesimal fineness 833)
18 carat (millesimal fineness 750)
16 carat (millesimal fineness 625)
14 carat (millesimal fineness 585)
10 carat (millesimal fineness 417)
9 carat (millesimal fineness 375)

1 Derivation
2 References
3 See also
4 External link

[edit] Derivation

The word derives from the Greek keration (fruit of the carob), via Arabic and Italian. Carob seeds were used as weights on precision scales because of their reputation for having a uniform weight; a 2006 study ^[1] by Lindsay Turnbull & others found this to not be the case - carob seeds have as much variation in their weights as other seeds^[2]. In the distant past, different countries each had their own carat, roughly equivalent to a carob seed. In the mid-16th century, it was adopted as a measure of gold purity, roughly equivalent to the Romn siliqua (one twenty-fourth of a golden solidus of Constantine I As a measure of diamond weight, from 1575. The Gk. measure was the equivalent of the Roman siliqua, which was ¹⁄₂₄ of a golden solidus of Constantine; but was likely never used to measure the weight for gold.^[3]