Somehow, the jewelry business (or, in British English, jewellery business) wound up with two homophones to classify the two materials it uses most, gold and gems. I mean, of course, carats and karats. If you’re writing or editing text about jewelry, you’ll need to keep these two straight. Here’s the difference:
Karat-with-a-K is a measure of gold fineness: 1 karat equals 1/24 pure gold in a metal alloy. A little simple math, then, tells us that pure gold is 24 karats (24/24ths = 100%).
A gold ring made of 34-karat gold would be 142% pure gold — an impossibility. If someone is trying to sell you a 34k gold ring, you’ll be lucky if it’s brass.
Carat-with-a-C is the unit of weight used with precious stones, 1 carat equaling 200 milligrams. One way to remember that carat-with-a-C goes with gems is to equate gems with crystals, which, like carat, starts with a C.
If that were all there is to it, this would be a fairly simple pair to keep clear. But many — perhaps most — dictionaries also list carat-with-a-C as an alternative spelling of karat-with-a-K. Some have supposed that this is the result of an acceptance of the common confusion of these two words, but when I did some research into how these homophones had been used historically, I uncovered some surprises that lead me to a different conclusion.
The first surprise was how few examples of karat I could find in English writing before 1900. I could, however, easily find numerous examples of carat used to measure gold, including in sources one would expect to be well-researched and authoritative, like this mineralogy textbook. That made me wonder if perhaps karat has only recently (in the last 100 years or so) become the norm for gold purity.
The second surprise — and this was the big one for me — is that both Webster’s New Twentieth Century Dictionary of the English Language (World Publishing Company, 1943) and Webster’s New International Dictionary, 2nd Edition Unabridged (Merriam-Webster, 1943 — the infamous “dord” dictionary) list karat only as another spelling of carat. If karat had long been the standard spelling for the measurement of gold purity, I would have expected at least one of these tomes to have made a note of it. But neither do.
My conclusion, then, is this: I don’t know what’s going on with karat. I’m no lexicographer, but I recognize that something is up here.
I also have a more useful conclusion: From an editorial standpoint, the distinction between carats and karats is a good one that we ought to maintain in the future. After all, expensive bling is made from both gold (karats) and gems (carats). But when we’re working with historical texts — or historical fiction even — perhaps we should be a bit less adamant about maintaining that separation, in the name of historical accuracy.