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,

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The carat system is increasingly being complemented or superseded by the millesimal fineness system in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy.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

Additional recommended knowledge

Guide to balance cleaning: 8 simple steps

Essential Laboratory Skills Guide

Weighing the right way

Derivation

The word carat is derived from the Greek kerátiōn (κεράτιων), “fruit of the carob”, via Arabic qīrāṭ (قيراط) and Italian. Carob seeds were used as weights on precision scales because of their reputation for having a uniform weight. However, a 2006 study^[1] by Lindsay Turnbull and 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, the carat was adopted as a measure of gold purity, roughly equivalent to the Roman siliqua ( of a golden solidus of Constantine I). As a measure of diamond weight, from 1575, the Greek measure was the equivalent of the Roman siliqua, which was of a golden solidus of Constantine; but was likely never used to measure the weight for gold.^[3]

Terminology

22/22K - a quality mark indicating the purity of gold. The first 22 signifies the "Skin purity" of gold jewellery and the second 22 signifies that after melting purity of the gold jewellery will be 22K (22 Karat) or 91.67% of pure gold. This symbol or stamp is very popular on the gold jewellery business in Asian countries like India, Sri Lanka, Pakistan, Bangladesh, Nepal, Yemen, and Gulf Countries.

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, and heightened consumer awareness made it a most sought-after stamp or quality mark.

Chuk Kam - In Chinese the term means pure gold. It is defined as 99.0% gold minimum with a 1.0% negative tolerance allowed.^[4][5] The quality of gold is guaranteed with a "Certificate of Gold" upon purchases in Hong Kong and Macau.

International caratages of gold jewellery

See also

• Gold as an investment
• Gold bar
• Gold coin
• Platinum coin

Notes

References

• Fallon, S. (2006) Hong Kong & Macau, 12th ed., Melbourne ; London : Lonely Planet, ISBN 1-7405-9843-1
• Harper, D. (2001) "Carat", in: Online Etymological Dictionary, accessed 28 August 2007
• New Scientist (2006) Did carob seeds allow shady diamond deals?, New Scientist magazine, 2550 (09 May), p. 20
• Turnbull, L.A., Santamaria, L., Martorell, T., Rallo, J. and Hector, A. (2006) "Seed size variability: from carob to carats", Biology Letters, 2 (3: Sept. 22), p. 397–400, DOI 10.1098/rsbl.2006.0476
• World Gold Council (2003) The Caratage (Karatage) System For Gold Jewellery, Online article accessed 28 August 2007