||It has been suggested that this article be merged with Millesimal fineness and Fineness. (Discuss) Proposed since April 2013.|
Carat purity is measured as 24 times the purity by mass:
- is the carat rating of the material,
- is the mass of pure gold or platinum in the material, and
- is the total mass of the material.
Therefore, 24-Carat gold is fine (actually 99.9% Au w/w rather than 100%), 18-Carat gold is 18 parts gold 6 parts another metal (forming an alloy with 75% gold), 12-Carat gold is 12 parts gold (12 parts another metal), and so forth.
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The Carat system is increasingly being complemented or superseded by the millesimal fineness system, in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy; e.g. 18-carat gold, 75% Au, would be called 750.
However, this system of calculation gives only the weight of pure gold contained in an alloy. The term 18-carat gold means that the alloy's weight consists of 75% of gold and 25% of alloy(s). The quantity of gold by volume in a less than 24-carat gold alloy differs according to the alloy(s) used. For example, knowing that standard 18-carat yellow gold consists of 75% gold, 12.5% silver and the remaining 12.5% of copper (all by weight), the volume of pure gold in this alloy will be 60% since gold is much more dense than the alloys used: 19.32 g/cm³ for gold, 10.49 g/cm³ for silver and 8.96 g/cm³ for copper.
This formula gives the amount of gold in cm³ or in ml in an alloy:
- is the volume of gold in cm³ or in ml,
- is the total mass of the alloy in grams, and
- is the carat purity of the alloy.
To have the percentage of the volume of gold in an alloy, divide the volume of gold in cm³ or in ml by the total volume of the alloy in cm³ or in ml.
For 10-carat gold, the gold volume in the alloy represents about 26% of the total volume for standard yellow gold. One should be aware of this, since talking about purity according to weight could lead to some misunderstandings; for many people, purity means volume.
"Karat" is a variant of "carat". First attested in English in the mid-15th century, the word "carat" came from Middle French carat, in turn from Italian carato, which came from Arabic qīrāṭ (قيراط), which came from Greek kerátion (κεράτιον) meaning carob seed (literally "small horn") (diminutive of κέρας - keras, "horn") and was a unit of weight though it was not likely used to measure gold in classical times.
In 309 CE, Roman Emperor Constantine I began to mint a new gold coin solidus that was 1⁄72 of a libra (Roman pound) of gold equal to a mass of 24 siliqua, where each siliqua (or carat) was 1⁄1728 of a libra. This is believed to be the origin of the value of the karat.
22/22K - a quality mark indicating the purity of gold most popularly used in India. This purity was adapted and practiced by the big jewellers and was later passed to jewel smiths. The first 22 signifies the "Skin purity", the purity of the top layer of the gold jewelry, and the second 22 signifies that after melting purity of the gold jewellery will be 22-karat, or 91.67% of pure gold. This system is used to show consistency in the quality of the gold.
This practice was pioneered and introduced in the early mid-1980s by Nemichand Bamalwa & Sons of Kolkata, India, sparking a revolution in India, as it forced jewellers to indicate correctly the after-melting purity. Heightened consumer awareness made it a most sought-after stamp or quality mark.
International caratages of gold jewellery
|Region||Typical caratage (fineness)|
|Arabic countries, Far East (China, Hong Kong, Taiwan), Malaysia, Indonesia||24-karat "Chuk Kam" (99.0% min)|
|Arabic countries, Bangladesh, India, Pakistan, Malaysia, and Sri Lanka||22-karat (91.6%)|
|Arabic countries in the Persian Gulf region||21-carat (87.5%), 18-karat (75.0%) in most Egypt|
|Europe - Southern / Portugal||19.2-karat (80.0%)|
|Europe - Southern / Mediterranean||18-karat (75.0%)|
|Europe - Northern Germany etc.||8- to 18-karat (33.3 - 75.0%)|
|Russia/former USSR||9- (37.5%) and 14-karat/old 583 and new 585 проба (58.5%)|
|United Kingdom||9-carat to 22 carat (37.5-91.6%)|
|United States, Canada||10-karat to 18-karat (41.7-75%)|
Chinese Gold Standards 
Zú jīn (Mandarin), Chuk Kam (Cantonese) (足金) means pure gold, literally "full gold". It is defined as 99.0% gold minimum with a 1.0% negative tolerance allowed. The quality of gold is guaranteed with a "Certificate of Gold" upon purchases in Hong Kong and Macau. The related term "千足" and "万足" meaning "thousand exact" and "ten thousand exact" is also used for purity of 99.9% and 99.99% respectively. This is because the impurity is at most 1 in 1,000 in the case of 99.9% or 1 in 10,000 for 99.99%.
United States 
The USA Federal Trade Commission (FTC) has standardized the karat markings used within its boundaries for almost 7 decades as of 2010. Under these regulations, items 10-karat or greater are to be stamped with either "K" or "Kt." Decimal markings are also an option under the CFTC regulations.
Under-karating is against the law in the United States of America. There are specific mandated consequences including fines, etc., based upon the severity of the infraction(s).
In addition, there is a set of tolerances to the required karat markings in the USA (always designated with a "K" and never a "C") depending upon the use of various soldering requirements when setting stones, mounting crowns, or creating prongs, for example.
See also 
Gold alloys 
- Harper, Douglas. "carat". Online Etymology Dictionary.
- κεράτιον, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
- Walter W. Skeat (1888), An Etymological Dictionary of the English Language
- κέρας, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
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- World Gold Council (2003)
- Fallon, (2006)
- Title 16: Commercial Practices: PART 23—GUIDES FOR THE JEWELRY, PRECIOUS METALS, AND PEWTER INDUSTRIES
- Fallon, S. (2006) Hong Kong & Macau, 12th ed., Melbourne; London: Lonely Planet, ISBN 1-74059-843-1
- New Scientist (2006) Did carob seeds allow shady diamond deals?, New Scientist magazine, 2550 (9 May), p. 20
- World Gold Council (2003) The Karatage System For Gold Jewellery, Online article accessed 28 August 2007