carat

This entry describes small units of mass, today typically used, for precious stones. For the measure of the fineness of gold, see karat.

For the German and Scandinavian units of mass, see karat.

1

The metric carat, 1907 &ndash present, a unit of mass used for weighing precious stones, = 200 milligrams = 3.086 troy grains¹. Adopted by the 4th CGPM in 1907, for “diamonds, fine pearls, and precious stones.” Also called the international diamond carat.

For diamonds, the carat is subdivided into 4 carat grains (which are not troy grains), and sometimes into 100 points. A 2¼ carat diamond would be a diamond of 250 points. Another unit, used only within the trade, is grainer; 1 grainer = 50 milligrams.

In the United States, use of the metric carat began on July 1, 1913; prior to that the value 205.3 milligrams was used. According to the current national standard in the United States², the carat is not to be used at all; milligrams should be used instead. This prohibition is a good example of a collision between the SI purists on the one hand and existing law and trade practices on the other.

In some areas the general introduction of the metric system affected the introduction of the metric carat in interesting ways. In Germany, a law of 17 May 1856 abolished the carat, and the carat was not mentioned in later weights and measures legislation. But German law did not outlaw the use of particular names for units, only requiring that any unit be referable to an approved standard. Since the new carat = 200 milligrams, and the kilogram was an approved standard, the metric carat was legal.

In the United Kingdom, the Weights and Measures Act of 1878 specified that precious stones could only be sold by troy ounces, that is, carats were not legal in trade.  The Weights and Measures (Metric System) Act of 1897, however, authorized metric units of mass, and by extension the metric carat. It was specifically authorized by an Order in Council of 14 Oct. 1913. The legality of the metric carat for trade in precious stones and pearls was reaffirmed by the Weights and Measures of Act of 1963.

1. G. F. Kunz.
The new international metric carat of 200 milligrams.
Transactions of the American Institute of Mining Engineers, (New York Meeting, February 1913).

Pages 1225 – 1245.

G. F. Kunz.
The New International Diamond Carat of 200 Milligrams.
Transactions of the American Institute of Mining Engineers, vol. 47, 1914. (Butte Meeting, August 1913).

Pages 748 – 769.

Circular No. 43, Bureau of Standards (Nov. 1, 1913).

“On and after July 1, 1913, the unit of weight for imported diamonds, pearls and other precious stones will be the metric carat of 200 milligrams.”

U. S. Treasury Dept. order, June 17, 1913

“The decision of the British (Government) Board of Trade to make the metric carat of 200 milligrams the standard carat of the United Kingdom is of interest to the jewelry manufacturers of Birmingham.”

Dept. of Commerce, Bureau of Foreign and Domestic Commerce.
Daily Consular and Trade Reports.
Nos. 75-151; Volume 2; April, May and June 1913.
Washington: U.S.G.P.O., 1913.
Page 1598. Report 149, June 27, 1913.

2. IEEE/ASTM SI 10™-2002.
American National Standard for Use of the International System of Units (SI): The Modern Metric System.
New York: IEEE, 30 December 2002.

See Section 3.3.3.

2

In 1877, principal merchants in the gem trade in London, Paris, and Amsterdam met and agreed on a standard value of 204.9624 milligrams.

U. S. National Bureau of Standards.
Jeweler's and Silversmith's Weights and Measures (2nd ed.) Circular 43.
Washington, DC: U.S.G.P.O., 1921.

United States National Bureau of Standards.
Miscellaneous Publication 233.
Washington, DC: U.S.G.P.O, 1960.

Page 6, footnote 10.

3

In Indonesia, ? – 20th¹ century, a unit of mass used for diamonds, approximately 205 milligrams.

1. United Nations, 1966.

4

In Iran , ? – 20th¹ century , a unit of mass, approximately 193.3 milligrams.

1. United Nations, 1966.

Ancient and medieval values of the carat

5

The Greek (κεράτιον,  keration) and Roman (siliqua) carat, a unit of mass, about 189 milligrams, originally the weight of a seed of the carob tree (Ceratonia siliqua, also known as St. John's Bread), from whose pods the familiar chocolate substitute is made. In classical times 1 siliqua = 3 barley corns = 4 wheat grains. (Twelve hundred years later, Johnson's Dictionary (1755) defined a carat as four grains.) In modern times, conventionally taken as about 189 milligrams, based on the weight of the solidus, though later scholars place it at 187 milligrams. 

From the early 4^th through the 10^th century, a gold coin, the solidus, played a preeminent role in Roman and Byzantine coinage. It weighed 24 siliqua, and from this the carat became identified with the fraction 1/24. 

E. Babelon.
Traité des monnaies grecgues et romaines. I^ère Partie. Théorie et doctrine.
Paris: Ernest Leroux, 1901.

Cols. 574-575. Includes references to earlier works.

Lindsay A. Turnbull, Luis Santamaria, Toni Martorell, John Rall and Andy Hector.
Seed size variability: from carob to carats.
Biology Letters vol. 2, pages 397-400 (2006) doi:101098/rsbl.2006.0476

Differs from earlier in applying modern statistical methods. Their most original contribution may be that they studied human subjects' ability to reject outlier seeds by visual observation: “Our study shows that people are remarkably good at selecting seeds by eye, and can discriminate differences in weight of around 5%. In this instance, human selection seems, unusually, to have been stabilizing rather than directional leading to a distribution of carat weights around the mean weight of carob seeds rather than at the higher end of the distribution.”

6

The Egyptian carat, about 196 milligrams. 

G. C. Miles.
Early Arabic glass weights and stamps.
Numismatic Notes and Monographs, no. 111.
New York: American Numismatic Society, 1948.

7

Syrian and Arabian carats, about  212 milligrams  (On the authority of Phillip Grierson, we treat the Syrian and Arabian carats as essentially the same. The following discussion is mainly due to Grierson.)  

reform of the coinage by the Caliph 'Abd al-Malik, who chose to make the weight of the new dinar the weight of the mithkal, a well known, pre-Islamic Arabic weight probably based on the weight of the Greek drachme, as represented by coins. As part of this reform, the carat was defined as 1/20th of a mitkal.

The increased mass meant that the unit no longer reflected the mass of ceratonia seeds, and required a redefinition of its relation to wheat and barley seeds: 1 carat = 4 barley corns = 5 wheat grains.

Philip Grierson.
The Monetary Reforms of 'Abd al-Malik: Their Metrological Basis and Their Financial Repercussions.
Journal of the Economic and Social History of the Orient. Vol. 3, no. 3 (Oct. 1960.) pages 241-264.

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Last revised: 27 July 2009.