Diamond-weighing kit, with weights labelled in grams and carats
Thecarat (ct) is aunit ofmass equal to 200 mg (0.00705 oz; 0.00643 ozt), which is used for measuringgemstones andpearls.The current definition, sometimes known as themetric carat, was adopted in 1907 at the FourthGeneral Conference on Weights and Measures,[1][2] and soon afterwards in many countries around the world.[i] The carat is divisible into 100points of 2 mg. Other subdivisions, and slightly different mass values, have been used in the past in different locations.
In terms ofdiamonds, aparagon is a flawless stone of at least 100 carats (20 g).[3]
TheANSIX.12 EDI standard abbreviation for the carat isCD.[4]
First attested in English in the mid-15th century, the wordcarat comes from Italiancarato, which comes fromArabic (qīrāṭ; قيراط), in turn borrowed fromGreekkerátion κεράτιον 'carob seed',[5][6][7] a diminutive ofkeras 'horn'.[8] It was a unit of weight, equal to 1/1728 (1/123) of a pound (seeMina (unit)).[6][9][5]
Carob seeds have been used throughout history to measure jewelry, because it was believed that there was little variance in their mass distribution.[10] However, this was a factual inaccuracy, as their mass varies about as much as seeds of other species.[11]
In the past, each country had its own carat. It was often used for weighinggold. Beginning in the 1570s, it was used to measure weights ofdiamonds.[5]
An 'international carat' of 205 milligrams was proposed in 1871 by the Syndical Chamber of Jewellers, etc., in Paris, and accepted in 1877 by the Syndical Chamber of Diamond Merchants in Paris. A metric carat of 200 milligrams is exactly one-fifth of a gram and had often been suggested in various countries,[2] and was finally proposed by the International Committee of Weights and Measures, and unanimously accepted[2] at the fourth sexennial General Conference of the Metric Convention held in Paris in October 1907. It was soon made compulsory by law in France, but uptake of the new carat was slower in England, where its use was allowed by theWeights and Measures (Metric System) Act of 1897.[12]
In theUnited Kingdom the originalBoard of Trade carat was exactly3+1647⁄9691grains (~3.170 grains = ~205 mg);[ii] in 1888, the Board of Trade carat was changed to exactly3+17⁄101 grains (~3.168 grains = ~205 mg).[iii] Despite it being a non-metric unit, a number of metric countries have used this unit for its limited range of application.
The Board of Trade carat was divisible into fourdiamond grains,[iv] but measurements were typically made in multiples of+1⁄64 carat.
There were also two varieties ofrefiners' carats once used in the United Kingdom—thepound carat and theounce carat.[v] Thepound troy was divisible into 24pound carats of 240 grains troy each; the pound carat was divisible into fourpound grains of 60 grains troy each; and the pound grain was divisible into fourpound quarters of 15 grains troy each. Likewise, theounce troy was divisible into 24ounce carats of 20 grains troy each; the ounce carat was divisible into fourounce grains of 5 grains troy each; and the ounce grain was divisible into fourounce quarters of1+1⁄4 grains troy each.[14]
Thesolidus was also aRoman weight unit. There is literary evidence that the weight of 72 coins of the type calledsolidus was exactly 1Roman pound, and that the weight of 1 solidus was 24 siliquae. The weight of a Roman pound is generally believed to have been 327.45 g or possibly up to 5 g less. Therefore, the metric equivalent of 1siliqua was approximately 189 mg. The Greeks had a similar unit of the same value.[15]
Goldfineness incarats comes from carats and grains of gold in a solidus of coin. The conversion rates 1 solidus = 24 carats, 1 carat = 4 grains still stand.[16] Woolhouse'sMeasures, Weights and Moneys of All Nations[17] gives gold fineness in carats of 4 grains, andsilver in troy pounds[17] of 12 troy ounces of 20 pennyweight each.[clarification needed]
^abLiddell, Henry George; Scott, Robert."κεράτιον".A Greek-English Lexicon.Archived from the original on 2022-12-30. Retrieved2021-02-20 – via Perseus.Tufts.edu.
^Skeat, Walter W. (1888)."carat".An Etymological Dictionary of the English Language. London: Henry Frowde. pp. 93–94.
^Liddell, Henry George; Scott, Robert."κέρας".A Greek-English Lexicon.Archived from the original on 2012-10-08. Retrieved2021-02-20 – via Perseus.Tufts.edu.
^"carat".Oxford Dictionaries. Archived fromthe original on August 24, 2010 – via oxforddictionaries.com.
^Naturski, Sebastian."Carat Weight".Your Diamond Teacher. Retrieved3 March 2017.
^Chaffers, William (1883).Hall Marks on Gold and Silver Plate (6th ed.). London: Bickers & Son.
^Grierson, Philip (1960). "The Monetary Reforms of 'Abd Al-Malik".Journal of the Economic and Social History of the Orient.3 (3):241–264.doi:10.1163/156852060X00098.
