- For other uses of the word carat, see Carat.
The carat is a unit of mass used for measuring gems and pearls. Currently a carat is defined as exactly 200 mg (0.007,055 oz, 3.086 grains). This definition, known as the metric carat, was adopted in 1907 at the Fourth General Conference on Weights and Measures, and soon afterwards in many countries around the world. It is universally used today. The carat is divisible into one hundred points of two milligrams each.
The word came to English from French, derived from the Greek kerátion (κεράτιον), “fruit of the carob”, via Arabic qīrāṭ (قيراط) and Italian carato. The Latin word for carat is siliqua. In past centuries, different countries each had their own carat unit, all roughly equivalent to the mass of a carob seed. These units were often used for weighing gold.
Carob seeds were used as weights on precision scales because of their reputation for having a uniform weight. However, a 2006 study found carob seeds to have as much variation in their weights as do other seeds, though it seems that it is easier than with other seeds to recognize particularly large or small specimens and remove them. Thus, the carob seed was used as a weight not because it was naturally more uniform in weight, but because it could be more easily standardized.
Historical definitions in the United Kingdom
Board of Trade carat
In the United Kingdom, before 1888, the Board of Trade carat was exactly grains; after 1887, the Board of Trade carat was exactly grains. Despite it 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. Similarly, 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 (carat) was also a Roman weight unit. There is literary evidence that the weight of 72 coins of the type called solidus 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 grams less. Therefore the metric equivalent of 1 solidus was approximately 189 mg. The Greeks had a similar unit of the same value.
The carat in Byzantine Egypt
A carob based weight unit was also used in Egypt in the Byzantine 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 Mohammad's time
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.
- The United States adopted the metric carat definition on July 1, 1913, the United Kingdom on 1 April 1914.
- The literal translation of κεράτιον is little horn, which describes the seed pod.
- Turnbull, Lindsay, et al. “Seed size variability: from carob to carats”
- “Did carob seeds allow shady diamond deals?”, New Scientist, page 20, 6 May 2006.
- The pre-1888 Board of Trade carat, of which there were exactly per ounce troy, was approximately 205.4094 mg.
- The post-1887 Board of Trade carat, of which there were exactly 151½ per ounce troy, was approximately 205.3035 mg.
- Unlike the modern carat, the Board of Trade carat was not used for measuring pearls; those were measured with pearl grains.
- The refiners’ carats were the offspring of the carat as a measure of fineness for gold.
- Chaffers, William. 1883. Hall Marks on Gold and Silver Plate. 6th edition. London: Bickers & Son.
- Grierson, Philip (1960). "The Monetary Reforms of'Abd Al-Malik". Journal of the Economic and Social History of the Orient. 3: p. 241–264. doi:10.1163/156852060X00098.
- A chart showing the size of various round diamond carat weights.
- A PDF chart showing the size of various fancy shaped diamond carat weights.
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