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a unit of weight in gemstones, 200 milligrams (about 3 grains of troy or avoirdupois weight).
Abbreviation: c., ct.
karat.
a unit for measuring the fineness of gold, pure gold being 24 karats fine.
Abbreviation: k., kt.
Contemporary Examples

The largest D color flawless diamond ever auctioned, the 101.73 carat jewel is expected to fetch at least $20 million.
‘Absolute Perfection’ Diamond Up for Auction William O’Connor May 13, 2013

This tea caddy is rather splendid Much of the pottery is gilded in 22 carat gold leaf.
New Royal China Alert Tom Sykes May 19, 2013

Historical Examples

The original of the carat used for weighing in India is a small bean.
Illustrations of Universal Progress Herbert Spencer

Three or four dollars a carat is a fair price at the present time.
Birds and Nature Vol. 11 No. 1 [January 1902] Various

He sold them at first by the carat at a rate that would amount to $560 a pound.
Creative Chemistry Edwin E. Slosson

The carat used in weighing diamonds is 31⁄6 grains (nearly).
Cooley’s Practical Receipts, Volume II Arnold Cooley

Thus 18 carat gold has one-fourth of alloy, and so on with lower qualities down to 12, which is in reality only gold by courtesy.
Encyclopaedia Britannica, 11th Edition, Volume 15, Slice 4 Various

The average weight of the native Burmese ruby is about one-eighth of a carat.
The Wonder Book of Knowledge Various

This identity must be considered as referring to the accuracy of half the thirty-second part of a carat.
A Dictionary of Arts, Manufactures and Mines Andrew Ure

To which Bill rejoined, “And Mulvy, you’re all gold—twenty-two carat.”
A Boy Knight Martin J. (Martin Jerome) Scott

noun
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
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
noun
(US & Canadian) a measure of the proportion of gold in an alloy, expressed as the number of parts of gold in 24 parts of the alloy Also spelt (in Britain and certain other countries) 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.
n.

variant of carat (q.v.). In U.S., karat is used for “proportion of fine gold in an alloy” and carat for “weight of a precious stone.”