a unit of weight in gemstones, 200 milligrams (about 3 grains of troy or avoirdupois weight). Abbreviation: c., ct.

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

carat, caret, carrot, karat. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
carat (ˈkærət)
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
[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 10th Edition
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Word Origin & History

1469, from M.Fr. carat, from It. carato, from Arabic qirat "pod, husk, weight of 4 grains," from Gk. keration "carob seed," lit. "little horn" dim. of keras "horn." Carob beans were a standard for weighing small quantities. As a measure of diamond weight, from 1575. The Gk. measure was the equivalent
of the Roman siliqua, which was one-twentyfourth of a golden solidus of Constantine; hence the word took on a sense of "a proportion of one twentyfourth" and became a measure of gold purity (1555). Eighteen carat gold is eighteen parts gold, six parts alloy. It is unlikely that the carat was ever a measure of weight for gold.
Online Etymology Dictionary, © 2010 Douglas Harper
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Encyclopedia Britannica


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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Example sentences
The diamond industry could charge double or triple the per carat price of these
  super-deep diamonds if properly marketed.
Also there is a company that takes a diamond seed and creates up to two carat
The basic unit for weighing gemstones is the carat, which is equal to one-fifth
  of a gram.
If the diamond's weight is described in decimal parts of a carat, the figure
  should be accurate to the last decimal place.
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