The Karat, the Carat, what they are and how they came to be

Over human history, we’ve used a ton of different measurements systems, with almost every culture having invented their own. Just in mass, you can find several dozens. This includes such colorfully named ones as the Dutch Cask, Oka, Pood and Pennyweight.

As the world has become more connected and there was an ever greater need for standardization (I’m sorry, but how many Poods to a Pennyweight?) a lot of these systems have fallen by the wayside and the majority (95 percent) has switched over to the decimally-based metric system.

Still, there are a few alternative ways of measurements which somehow manage to hold out. No, I’m not talking about the Imperial System (though to somebody who grew up with the metric system that is an odd system to want hold on to).

Instead, here I’m going to discuss the Karat and the Carat.

No, they have nothing to do with carrots

In fact, they don’t have anything to do with vegetables (though there is a relationship to seeds, which we’ll cover later). Heck, they don’t even measure the same thing! Instead, the Karat is used to measure precious metal purity while gem carats measure weight, just like grams and ounces do.

Let’s look at precious-metal first. Interesting fact, only in the US is this karat spelled with a ‘k’. In the rest of the world, it is also spelled with a ‘c’, just like the gemstone carat. As if giving two measurement systems almost the same name isn’t confusing enough! In order to help us tell them apart, we’ll use the American spelling here.

The karat measure purity, with 24 karats meaning something has no extra metals mixed in and is 100% pure. That means that each karat is about 4.17 percent (I say ‘about’ because it would really be 4.16 with the six recurring). This means that 12 karats is half pure, while 18 karats is 3/4 pure.

As for gem carats, here it’s not about purity but instead, they measure weight. Each carat weighs 200 milligrams. That’s .007 ounces for the remaining 5% of the world.

So, to recap quickly: If somebody gives you a piece of jewelry that’s 2 karats, go ahead and get really offended. That’s not even 10% pure! On the other hand, if somebody gives you a two-carat diamond you should feel quite flattered, as a stone that big retails for somewhere in the $5,000 to $60,000 range.

Why do they use the same name but measure different things?

Back in yea olden days, measurement was difficult. For a long time, there was no centrally located measurement bureau that decided how much a certain measurement – like the pound – weighed. Even when there was, carrying the weights all over the world on horseback would have been difficult indeed.

The solution merchants came up with was to find something that weighed the same everywhere. People somehow got it into their heads that the carob seed was such a thing and that it varied less in weight than other seeds did. This turns out not to be true, these seeds vary just as much as other seeds in weight. But then belief often trumps reality.

And so, Carats varied widely, with one weighing 187 milligrams in Cyprus and 205.649 milligrams in South Africa. Obviously, that led to all sorts of problems. For that reason, in 1907, at the Fourth General Conference on Weights and Measures, the weight was standardized as the metric carat which weighed 200 milligrams.

The precious-metal karat, in the meantime, reputedly comes from medieval times in Germany. There they had a gold coin called the Mark, which had a standardized weight of 24 carobs. The problem was, as anybody who has ever had the opportunity to play with gold before knows, pure gold is too soft to use this way. You’d end up with bent coins and defaced monarchs!

For that reason, the custom was to add copper to create a harder alloy. Of course, as copper is less valuable than gold, people wanted to know the relationship of gold to copper. The people who minted the coin decided to state how much gold there was in the gold by giving the karat weight of the gold only. From there it then became associated with purity, with a 22 karat gold mark being 91.667 percent pure.

So there you have it. That’s the story of the karat, the carat and how they came to be. Now, you’ll have a great story to tell when somebody next shows off their diamond ring and drops in how many carats it is!

Want to learn more about how the number of carats affects the value of a diamond? Check out our diamond grading guide for more information!

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