Why Are Gems and Gold Measured by Carats and Karats?

May 30, 2023

Diamond that can be measured in carats or karats.

The words carat and karat are similar-sounding words, but they refer to two entirely different things. The carat is the standard unit of measurement for the weight of gemstones, while a karat is used to indicate the proportion of gold in an alloy.

The weight of a carat is 200 milligrams or 0.2 grams, and its weight was standardized at an international conference in 1907. But why is it called a carat?

Hundreds of years ago, people had no standard for the weight of gems, so they used the carob seed of the carob tree as a base unit of weight. One seed was designated as weighing one carat. The weight of the gem could then be determined by the number of carob seeds it equaled in weight since, at the time, it was believed that carob seeds had a uniform weight.

But there was a problem with this system of measures based on the carob seed. There are variations in the weight of each seed from a single pod of the carob tree. So how did it become such a relied upon method?

A study in 2006 from the Royal Society: Biology Letters looked at the problem. They discovered that human selection of uniform seeds might have played more of a role in using the seeds as a measure than the supposed uniform weight of the seeds themselves. They found that people could differentiate the differences in the weight of the seeds by around 5%, just by using their eyes. That meant that everyone at the bargaining table left happy, knowing they were using roughly the same weight to measure their jewels. Of course, it also meant the system could be abused.

There is also the karat, or the measurement of the proportion of gold, which is called a karat in the United States, but outside the US, it is spelled “carat.” This spelling of the measurement for gold dates back to the 16th century but should not to be confused with the unit of weight measure for gemstones.

One karat here means that there is 1/24 pure gold in an alloy. This measure of gold purity came about during the time of Emperor Constantine and was also based on the carob seed. A solid gold coin weighed 24 karats or the equivalent weight of 24 carob seeds. Today, it is a measure of the purity of gold, so a 24-karat gold ring is pure gold, while a 14-karat ring is 14 parts gold along with ten parts of some other metal.

There two more words that sound like carat and karat. It is called the “caret” (note the change to “e”). It is the proofreading mark that looks like a triangle with the bottom missing (^) that has nothing to do with gems or gold. It is an indicator where text needs to be inserted. And things wouldn’t be complete without a mention of the most famous of these homophones, the “carrot.”

About the author 

Daniel Ganninger - The writer, editor, and chief lackey of Knowledge Stew, the author of the Knowledge Stew line of trivia books, and editor of Fact World and the Knowledge Stew sister site on Medium, our ad-free subscription sites. I hope you learn many new things here that add to your knowledge.

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