Carat (mass)
Encyclopedia
The carat is a unit
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...

of mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

equal to 200 mg
Gram
The gram is a metric system unit of mass....

(0.2 g
Gram
The gram is a metric system unit of mass....

; 0.007055 oz
Ounce
The ounce is a unit of mass with several definitions, the most commonly used of which are equal to approximately 28 grams. The ounce is used in a number of different systems, including various systems of mass that form part of the imperial and United States customary systems...

) and is used for measuring gemstone
Gemstone
A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...

s and pearl
Pearl
A pearl is a hard object produced within the soft tissue of a living shelled mollusk. Just like the shell of a mollusk, a pearl is made up of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other...

s.
The current definition, sometimes known as the metric carat, was adopted in 1907 at the Fourth General Conference on Weights and Measures
General Conference on Weights and Measures
The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures . It is one of the three organizations established to maintain the International System of Units under the terms of the Convention du Mètre of 1875...

, and soon afterward in many countries around the world. The carat is divisible into one hundred points of two milligrams each. Other subdivisions, and slightly different mass values, have been used in the past in different locations.

In terms of diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

s, a paragon
Paragon
A paragon is a large, flawless diamond. The title is now used figuratively to denote a model of excellence or perfection of any kind; one having no equal; a perfect embodiment of a concept.Paragon may also refer to:-Places:...

is a flawless stone of at least 100 carats (20 g).

The ANSI
American National Standards Institute
The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...

X.12 EDI
Electronic Data Interchange
Electronic data interchange is the structured transmission of data between organizations by electronic means. It is used to transfer electronic documents or business data from one computer system to another computer system, i.e...

standard abbreviation for the carat is CD.

## Etymology

First attested in English in the mid-15th century, the word carat came from Middle French
Middle French
Middle French is a historical division of the French language that covers the period from 1340 to 1611. It is a period of transition during which:...

carat, in turn from Italian carato, which came from Arabic
Arabic language
Arabic is a name applied to the descendants of the Classical Arabic language of the 6th century AD, used most prominently in the Quran, the Islamic Holy Book...

qīrāṭ (قيراط), which came from Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

kerátion (κεράτιον) meaning carob seed (literally "small horn")
(diminutive of κέρας - keras, "horn")
and was a unit of weight
though it was not likely used to measure gold in classical times.
The Latin word for carat is siliqua
Siliqua
The siliqua is the modern name given to small, thin, Roman silver coins produced from 4th century and later. When the coins were in circulation, the Latin word siliqua was a unit of weight defined as one-twentyfourth of the weight of a Roman solidus .The term siliqua comes from the siliqua graeca,...

.
This common belief that carat derives from carob seeds stems from the assumption that the seeds had unusually low variability in mass. However, it has been found that carob seeds had average variability compared to other seeds. This was not the only reason. It is said that, in order to keep regional buyers and sellers of gold honest, potential customers could retrieve their own carob seeds on their way to the market, to check the tolerances of the seeds used by the merchant. If this precaution was not taken, the potential customers would be at the mercy of "2 sets of carob seeds". One set of "heavier" carob seeds would be used when buying from a customer (making the seller's gold appear to be less). Another, lighter set of carob seeds would be used when the merchant wanted to sell to a customer.

In the past, each country had its own carat. It was often used for weighing gold. Starting in the 1570s, it was used to measure weights of diamonds.

In the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

, before 1888, the Board of Trade carat was exactly (≈ 3.170) grains; after 1887, the Board of Trade
The Board of Trade is a committee of the Privy Council of the United Kingdom, originating as a committee of inquiry in the 17th century and evolving gradually into a government department with a diverse range of functions...

carat was exactly (≈ 3.168) grains. Despite its being a non-metric unit, a number of metric countries used this unit for its limited range of application.

The Board of Trade carat was divisible into four diamond grains, but measurements were typically made in multiples of carat.

### Pound carat and ounce carat

There were also two varieties of refiners’ carats once used in the United Kingdom — the pound carat and the ounce carat. 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. Likewise, 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.

## The carat of the Romans and Greeks

The solidus was also a Roman weight unit. There is literary evidence that the weight of 72 coins of the type called solidus
Solidus (coin)
The solidus was originally a gold coin issued by the Romans, and a weight measure for gold more generally, corresponding to 4.5 grams.-Roman and Byzantine coinage:...

was exactly a Roman pound, and that the weight of a 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 1 siliqua was approximately 189 mg. The Greeks had a similar unit of the same value.

Gold fineness in carats comes from carats and grains of gold in a solidus of coin. One solidus = 24 carats, 1 carat = 4 grains, is preserved right up to this day. A book gives gold fineness in carats of 4 grains, and silver in (pound) of 12 ounces each 20 dwt.

## The carat in Byzantine Egypt

A carob-based weight unit was also used in Egypt in the Byzantine
Byzantine
Byzantine usually refers to the Roman Empire during the Middle Ages.Byzantine may also refer to:* A citizen of the Byzantine Empire, or native Greek during the Middle Ages...

and early Arab periods. In this region, glass weights were used for weighing coins. From these, the weight of the Egypt carat has been reconstructed as 196 mg. This is consistent with the average weights of carob seeds in the region.

## The Syrian and Arabic carat in the First Millennium CE

According to literary sources, the Arabic carat was only 2% less than the Syrian carat. Based on coins and glass weights, their weight was reconstructed as approximately 212 mg. This is consistent with literary information that a solidus weighed slightly less than 22 carats.