Carat (purity)

From Wikipedia, the free encyclopedia
Jump to: navigation, search
For the unit of mass of the same name, see Carat (mass).

The carat or karat (symbol: K or kt) is a unit of purity for gold alloys.


Karat purity is measured as 24 times the pure mass divided by the total mass:

K = 24\,\frac{M_\text{g}}{M_\text{m}}


K is the carat rating of the material,
M_\text{g} is the mass of pure gold in the alloy, and
M_\text{m} is the total mass of the material.

Therefore, 24-karat gold is fine (also includes down to 99.95% gold by mass), 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.[1]

In England, the karat was divisible into four grains, and the grain was divisible into four quarts. For example, a gold alloy of \tfrac{381}{384} fineness (that is, 99.2% purity) could have been described as being 23-karat, 3-grain, 1-quart gold.

The karat system is increasingly being complemented or superseded by the millesimal fineness system, in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy; e.g. 18-karat gold, 75% Au, would be called 750.

Karat conversion:
58.33% - 62.50% = 14k (acclaimed 58.33%)
75.00% - 79.16% = 18k (acclaimed 75.00%)
91.66% - 95.83% = 22k (acclaimed 91.66%)
95.83% - 99.95% = 23k (acclaimed 95.83%)
99.95 and above = 24k (acclaimed 99.99%)


However, this system of calculation gives only the mass of pure gold contained in an alloy. The term 18-carat gold means that the alloy's mass 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 mass), the volume of pure gold in this alloy will be 60% since gold is much denser than the other metals 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 cubic centimetres or in millilitres in an alloy:

V_\text{Au} = \frac{M_\text{a} \times \tfrac{kt}{24}}{19.32}


V_\text{Au} is the volume of gold in cubic centimetres or in millilitres,
M_\text{a} is the total mass of the alloy in grams, and
kt 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 cubic centimetres or in millilitres by the total volume of the alloy in cubic centimetres or in millilitres.

For 10-carat gold, the gold volume in the alloy represents about 26% of the total volume for standard yellow gold. Talking about purity according to mass 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")[2][3][4] (diminutive of κέρας - keras, "horn"[5]) and was a unit of mass[6] though it was probably not used to measure gold in classical times.[2]

In 309 CE, Roman Emperor Constantine I began to mint a new gold coin solidus that was 172 of a libra (Roman pound) of gold[7] equal to a mass of 24 siliqua, where each siliqua (or carat) was 11728 of a libra.[8] This is believed to be the origin of the value of the karat.[9]


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.

This symbol or stamp can be found in such Asian countries as India, Sri Lanka, Pakistan, Bangladesh, Nepal, Yemen, and Persian Gulf countries.

International caratages of gold jewellery

Region[10] 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, Nepal, and Sri Lanka 22-karat (91.6%)
Arabic countries in the Persian Gulf region 21-carat (87.5%)
Europe - Southern / Portugal 19.2-karat (80.0%)
Europe - Southern / Mediterranean 18-karat (75.0%)
Europe - Northern Germany, Scandinavia 8 to 18-karat (33.3-75.0%)
Brazil, Egypt 18-karat (75.0%)
Russia and the rest of the former Soviet Union 9 (37.5%) and 14 (58.3%) karat/old and new 14.04-karat (58.5%) 585 проба
United Kingdom 9-carat to 22-carat (37.5-91.6%)
United States, Canada 10-karat to 18-karat (41.7-75%)
Turkey 14-karat to 22-karat (58.3-91.6%)

Chinese gold standards[edit]

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.[10][11] 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[edit]

The US Federal Trade Commission (FTC) has standardized the karat markings used within its boundaries since the 1940s. 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. 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 United States (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.[12]

See also[edit]

Gold alloys[edit]


  1. ^ Comprehensive Jewelry Precious Metals Overview International Gem Society (IGS), Retrieved 01-16-2015
  2. ^ a b Harper, Douglas. "carat". Online Etymology Dictionary. 
  3. ^ κεράτιον, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  4. ^ Walter W. Skeat (1888), An Etymological Dictionary of the English Language
  5. ^ κέρας, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  6. ^ carat, Oxford Dictionaries
  7. ^ Vagi, David L. (1999). Coinage and History of the Roman Empire. II: Coinage. Chicago: Fitzroy Dearborn. p. 100. ISBN 1-57958-316-4. Retrieved 18 November 2011. 
  8. ^ Grierson, Philip (1968). Catalogue of the Byzantine Coins in the Dumbarton Oaks Collection and in the Whittemore Collection. 2: pt. 1. Washington, DC: Dumbarton Oaks. p. 8. ISBN 0-88402-024-X. Retrieved 18 November 2011. 
  9. ^ Turnbull, L. A.; Santamaria, L.; Martorell, T.; Rallo, J.; Hector, A. (2006). "Seed size variability: From carob to carats". Biology Letters 2 (3): 397–400. doi:10.1098/rsbl.2006.0476. PMC 1686184. PMID 17148413.  edit
  10. ^ a b World Gold Council (2003)
  11. ^ Fallon, (2006)