Carat (mass)

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For other uses of the word carat, see Carat.

The carat is a unit of mass used for measuring gems and pearls, and is exactly 200 milligrams. The word came to English from French, derived from the Greek kerátion (κεράτιον), “fruit of the carob”, via Arabic qīrāṭ (قيراط) and Italian carato. Carob seeds were used as weights on precision scales because of their reputation for having a uniform weight. However, a 2006 study found carob seeds to have as much variation in their weights as do other seeds, though it seems that it is easier than with other seeds to recognize particularly large or small specimens and remove them. Thus, the carob seed was used as a weight not because it was naturally more uniform in weight, but because it could be more easily standardized.

In past centuries, different countries each had their own carat unit, all roughly equivalent to the mass of a carob seed. In the United Kingdom, before 1888, the Board of Trade carat was exactly $3,tfrac{1647}{9691}$ grains; after 1887, the Board of Trade carat was exactly $3,tfrac{17}{101}$ grains. Despite it being a non-metric unit, a number of metric countries used this unit for its limited range of application. In 1907, the definition of the metric carat of 200 milligrams was adopted at the Fourth General Conference on Weights and Measures; this replaced the non-metric carat definition in the United States on July 1, 1913, and superseded the Board of Trade carat in the United Kingdom on 1 April 1914. The metric carat is the one that is universally used today.

The carat is divisible into one hundred points of two milligrams each. The Board of Trade carat was divisible into four diamond grains, but measurements were typically made in multiples of $tfrac{1}{64}$ carat.

There were also two varieties of refiners’ carats once used in the United Kingdom — the pound carat and the ounce carat. The pound troy was divisible into 24 pound carats of 240 grains troy each; the pound carat was divisible into four pound grains of 60 grains troy each; and the pound grain was divisible into four pound quarters of 15 grains troy each. Similarly, the ounce troy was divisible into 24 ounce carats of 20 grains troy each; the ounce carat was divisible into four ounce grains of 5 grains troy each; and the ounce grain was divisible into four ounce quarters of 1¼ grains troy each.

For diamonds, a paragon is a flawless stone of at least 100 carats (20 g).

The ANSI X.12 EDI standard abbreviation for the carat is CD.

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Last updated on Tuesday February 05, 2008 at 12:54:43 PST (GMT -0800)
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