carat

For the measure of the fineness of gold, see karat.

1

A unit of mass used for weighing precious stones, currently = 200 milligrams = 3.086 grains¹. Adopted by the 4th CGPM in 1907, for “diamonds, fine pearls, and precious stones.” Sometimes called the metric carat. See history of the carat.

For diamonds, the carat is sometimes subdivided 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, the value 205.3 milligrams was used prior to July 1, 1913; thereafter the metric carat has been used. In the United Kingdom, the metric carat was made legal for precious stones and pearls by the Weights and Measures of Act of 1963.

According to the current national standard in the United States², the carat is not to be used; 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.

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.

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

1. United Nations, 1966.

3

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

1. United Nations, 1966.

History of the carat

The carat is an ancient unit, 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 the seed was known as the siliqua or keratia, and one siliqua = three barley corns = four wheat grains. Twelve hundred years later, Johnson's Dictionary (1755) defined a carat as four grains.

By the middle of the 19th century various values were being used for the carat as a measure of the mass of precious stones:

Amsterdam	205.1 milligrams
London	205.3 milligrams
Lisbon and Rio de Janeiro (the quilate)	199.1 milligrams
Venice	207.0 milligrams

In 1877, principal merchants in the gem trade in London, Paris, and Amsterdam met and agreed on a standard value of 205 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.