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carat

[kar-uh t] /ˈkær ət/
noun
1.
a unit of weight in gemstones, 200 milligrams (about 3 grains of troy or avoirdupois weight).
Abbreviation: c., ct.
2.
Origin
1545-1555
1545-55; < Medieval Latin carratus (used by alchemists) < Arabic qīrāṭ weight of 4 grains < Greek kerátion carob bean, weight of 3.333 grains, literally, little horn, equivalent to kerat- (stem of kéras) horn + -ion diminutive suffix
Can be confused
carat, caret, carrot, karat.
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for carats

carat

/ˈkærət/
noun
1.
a measure of the weight of precious stones, esp diamonds. It was formerly defined as 3.17 grains, but the international carat is now standardized as 0.20 grams
2.
Usual US spelling karat. a measure of the proportion of gold in an alloy, expressed as the number of parts of gold in 24 parts of the alloy
Word Origin
C16: from Old French, from Medieval Latin carratus, from Arabic qīrāt weight of four grains, carat, from Greek keration a little horn, from keras horn
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for carats

carat

n.

also karat, mid-15c., from Middle French carat "measure of the fineness of gold" (14c.), from Italian carato or Medieval Latin carratus, both from Arabic qirat "fruit of the carob tree," also "weight of 4 grains," from Greek keration "carob seed," also the name of a small weight of measure (one-third obol), literally "little horn" diminutive of keras "horn" (see kerato-).

Carob beans were a standard for weighing small quantities. As a measure of diamond weight, from 1570s in English. The Greek measure was the equivalent of the Roman siliqua, which was one-twentyfourth of a golden solidus of Constantine; hence karat took on a sense of "a proportion of one twentyfourth" and became a measure of gold purity (1550s). Eighteen carat gold is eighteen parts gold, six parts alloy. It is unlikely that the classical carat ever was a measure of weight for gold.

Online Etymology Dictionary, © 2010 Douglas Harper
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Encyclopedia Article for carats

carat

unit of weight for diamonds and certain other precious gems. Before 1913 the weight of a carat varied in different gem centres. Originally based on the weight of grains or leguminous seeds, which, of course, varied in size from place to place, the carat was equivalent to 0.2053 gram (3.168 troy grains) in London, 0.1972 g in Florence, and 0.2057 g in Amsterdam. The weight of a gemstone was calculated in terms of whole carats plus fractions (12, 14, 18, 116, 132, or 164) of a carat; thus, a stone might be said to weigh 3 + 14 + 116 carats. After various unsuccessful attempts to standardize the carat, the metric carat, equal to 0.200 g, and the point, equal to 0.01 carat, were adopted by the United States in 1913 and subsequently by most other countries. The weights of diamond, ruby, sapphire, emerald, topaz, aquamarine, garnet, tourmaline, zircon, spinel, and sometimes opal and pearl are expressed in carats

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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