What is a karat? It is a way to measure the purity of gold. (In the UK, karat is spelled as carat – more on that later). However, a carat is a unit of mass that is 200 mg. It is used for measuring gemstones and pearls. Both Karats and carats are not used to measure silver. Let’s look at karats as a measure of the purity of gold and then look at carats used to measure gems.
We need to look at where the word comes from first. Carat came into English from the French “carat”, which came from the Arabic “qirat”, which came from the Greek “keration”. A keration was the fruit of the carob tree. A carob tree is a flowering tree that grows in the Mediterranean and Middle East. It produces fruit that look like seed pods and are edible. In ancient times, carob seeds were used to weigh small quantities of things. The seeds were chosen because it was said that all of the seeds were exactly the same size. This is not true and can be proven to be not true if one were to just get a handful of random carob fruit seeds. However, for the purpose of measurement, the idea that they were all the same size was enforced.
So, why do we use karats to measure the purity of gold? When we are using karats to measure the purity of gold, we are not talking about weight, at least, not these days. Pure gold is taken to be 24 karats. When gold is sold, to reduce the price, it is quite often mixed with other metals, such as copper. When that happens, the purity is reduced and that purity is measured as 1/24th of pure gold. If you buy a ring that is 14-karat gold, that means the ring has 14/24 parts of gold and 10/24 of some other metal, most likely copper. If you buy a ring that is 24-karat gold, it is pure gold with no other metals added in. Gold tends not to go lower than 10-karat because then there would not be enough purity left.
Where does the 1/24 system come from? It comes from the Roman system of weighing and a coin that was introduced by Emperor Constantine. The Romans inherited a lot of the Greek measurements and the keration was one of them. In Greek, it meant the fruit of the carob tree, which was a measurement. The Romans took this measurement and turned it into Latin, which is siliqua graeca. Emperor Constantine made a new coin to replace a coin called the aurerus in the 4th century. His coin was called the solidi and one solidus weighed 1/72 of a pound of gold. One solidus was then divided into 24 siliquae, from the word siliqua graeca. So, 24 siliqua graeca (seed of the fruit of the carob tree) were worth 1 solidus, which was pure gold.
The word karat came into English from the Arabic word for the fruit of the carob tree, which was qirat. This came into Italian and French, before making its way to English.
The second meaning of carat, although spelled with a c and not a k, is the weight of a gem. This goes back to the days when people used the seeds from the carob tree to weigh things. The idea was that all of the seeds were the same weight and it is a foolproof measuring system, but the seeds are obviously different sizes. It probably ended up as a negotiation between buyer and seller where they would have to agree that all of the seeds were the same size. These days, a carat officially weighs 200 mg. In Roman times, they would have weighed slightly different amounts depending on the seed that was used. If you have a diamond that is 10 carats, that means it weighs 2 g, which is very large.
So, where is the connection between karats as a measure of purity and carats as a measure of weight? That seems to come down to the Romans again. Their siliqua coin was 1/24th of a solidus and it also weighed the same weight as a carob seed, which is 1 carat. And this is what I learned today.
Photo by Pixabay: https://www.pexels.com/photo/gold-bar-lot-47047/
Sources
https://www.tiffany.com/engagement/the-tiffany-guide-to-diamonds/carat/
https://www.etymonline.com/word/carat
https://www.merriam-webster.com/grammar/usage-carat-vs-karat
https://en.wikipedia.org/wiki/Fineness#Karat
https://en.wikipedia.org/wiki/Carob
https://en.wikipedia.org/wiki/Carat_(mass)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1686184/
https://en.wikipedia.org/wiki/Siliqua
https://science.howstuffworks.com/dictionary/geology-terms/question64.htm