The terms karats and carats both refer to bling in the jewelry trade and in geology, but the two words don’t mean the same thing.
What’s the difference between karats and carats? Are they the same?
A karat is a term that describes the purity of gold. A karat is 1/24th of a portion, so 24 karat gold is pure gold. 14-karat gold is 14 parts gold and 10 parts other metals. 18-karat gold is 18/24 parts gold or 75% gold. Pure gold is very soft, so it’s usually mixed with other metals to form an alloy that’s hard, less expensive, and sometimes has a different color. The abbreviation for karat is K or k. For example, “14k gold” refers to 14 karat gold.
A carat is a unit of mass used in the jewelry industry that describes the weight of diamonds and other gemstones. One carat equals 200 milligrams or 0.2 grams. To put this into perspective, a pound is 453 grams or 2264 carats. A 2-carat diamond weighs 400 mg. Gemstones have different densities, so 1 carat of one stone might be larger or smaller in volume than 1 carat of another material. For example, a 1-carat diamond is slightly smaller than a 1-carat moissanite, assuming both stones share the same cut. A carat may be divided into 100 points, each with a mass of 2 milligrams. The abbreviation for carats is CT or ct.
History of the Terms Carat and Karat
The word carat comes from a Greek word, meaning “carob seed”. The seeds were units of weight for small objects, as measured on balances. The carat came into use for measuring the weight of diamonds in the 1570s. While some people surmise carob seeds were used because they were uniform in weight, this isn’t actually true. The size and mass of these seeds varies about as much as for other species. Until 1907, the mass of a carat varied from one country to another, ranging from 187 mg in Cyprus to 216 mg in Livorno. The unit was standardized to become the 200 mg metric carat, which is the unit used today throughout the world.
“Karat” is a variant of the word “carat”. There is no record of carob seeds ever measuring gold. At some point, the gemstone mass term and gold purity term diverged. In 309 CE, the Roman emperor Constantine I minted gold coins that had the mass of 24 siliqua, where a siliqua or carat was 1/1728 of a libra or pound. This is probably the origin of the use of 24 parts for karats. The term karat is only used to describe purity or fineness of gold. It is 24 times the pure mass divided by the total mass:
K = 24 x (Mg/Mm)
where K is the karat rating, Mg is mass of gold, and Mm is total mass.
Did You Know?
- A “paragon” is a flawless diamond weighing at least 100 carats.
- The ANSI X.12 EDI standard abbreviation for the unit carat is CD.
- Millesimal fineness is another unit used to measure purity of gold. The term also applies to other precious metals, such as silver and platinum. The term refers to parts per thousands. So, 18k gold, which is 75% gold or 750 parts per 1000, is called “750”. Usually, millesimal fineness is rounded to three figures, but it also describes high purity. For example, extremely pure gold that is 99.99% pure, is “9999” or “four nines fine”. The finest gold ever produced was six nines fine (refined by the Perth Mint in 1957).
References
- Turnbull, L. A.; Santamaria, L.; Martorell, T.; Rallo, J.; Hector, A. (2006). “Seed size variability: From carob to carats”. Biology Letters. 2 (3): 397–400. doi:10.1098/rsbl.2006.0476
- Vagi, David L. (1999). Coinage and History of the Roman Empire. Vol. II: Coinage. Chicago: Fitzroy Dearborn. ISBN 978-1-57958-316-3.
- Zhengzhang, Tao (July 1991). “On the origin of the carat as the unit of weight for gemstones”. Chinese Journal of Geochemistry. 10 (3): 288–293. doi:10.1007/BF02843332