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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.^[1] The word came to English from French, derived from the Greek kerátion (κεράτιον), “fruit of the carob”,^[2] 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^[3] 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.^[4] 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 grains;^[5] after 1887, the Board of Trade carat was exactly grains.^[6] 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on Weights and Measures; this replaced the non-metric carat definition in the United States on July 1, 1913, and superseded the Board of Trade carat in the United Kingdom on 1 April 1914.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,^[7] but measurements were typically made in multiples of carat.

There were also two varieties of refiners’ carats once used in the United Kingdom — the pound carat and the ounce carat.^[8] 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.^[9]

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.

1. ^ Two hundred milligrams is approximately grains.
2. ^ The literal translation of κεράτιον is little horn, which describes the seed pod.
3. ^ Turnbull, Lindsay, et al. “Seed size variability: from carob to carats”
4. ^ “Did carob seeds allow shady diamond deals?”, New Scientist, page 20, 6 May 2006.
5. ^ The pre-1888 Board of Trade carat, of which there were exactly per ounce troy, was approximately 205.4094 mg.
6. ^ The post-1887 Board of Trade carat, of which there were exactly 151½ per ounce troy, was approximately 205.3035 mg.
7. ^ Unlike the modern carat, the Board of Trade carat was not used for measuring pearls; those were measured with pearl grains.
8. ^ The refiners’ carats were the offspring of the carat as a measure of fineness for gold.
9. ^ Chaffers, William. 1883. Hall Marks on Gold and Silver Plate. 6^th edition. London: Bickers & Son.

• A chart showing the size of various round diamond carat weights.
• A PDF chart showing the size of various fancy shaped diamond carat weights.