If you have a piece of gold jewelry, grab it and a magnifier and see if you can find the marking that tells you how “gold” it is.
Does it say “10k,” “14k,” “18k,” or some variation? Can you find one that says “14c”? Probably not.
Most people know that the “k” stands for “karat,” a measure of purity. Something that is 100 percent gold is 24 “karat”; something that is 12 “karat” is 50 percent gold, the rest being impurities or harder metals like silver, bronze, or brass. (Because pure gold is so soft, you rarely see 24-karat gold jewelry, and even the “purest” gold is at most 99.90 percent gold.)
Many people, especially journalists, are taught that “karat” is used only for gold. For the weight of gemstones, “carat” is the measurement: A “carat” is equal to 200 milligrams. (And 100 proof alcohol is 50 percent alcohol. Wouldn’t life be simpler if we didn’t require all these conversions?)
Of the two terms, “carat” is the older; the Oxford English Dictionary traces it to 1552 in English as a measurement for the fineness of gold. By 1568, it was also a measurement for the weight of diamonds and other precious stones, though its first appearance spelled it “kirat.” (It’s also been spelled “carrets,” “caracts,” “caracks,” etc. At least some spelling has been standardized.)
It seems appropriate that we now have “chocolate diamonds,” since the original “carat” was also derived from a carob bean. As Merriam-Webster notes, the word that became “carat” came from “Greek keration, referring to both the carob bean and a small weight.” Just as carob beans vary in weight, so did the “standard” weight of a “carat” vary, from “187 mg in Cyprus to 216 mg in Livorno,” as a science site says. The 200-milligram standard was established in 1907, though it took a few more years for it to be adopted universally.
Aside from the variant spelling for “carat” early on, “karat” did not materialize in English until 1901, the OED says, when it first appeared in a jewelers magazine, leading to speculation that it was a public relations ploy to differentiate gold value from gemstone value. (The Online Etymology Dictionary traces it earlier, to 1854.) But O. Henry used “karat” to refer to a diamond in a story that appeared in 1903.
(English also has “caret” [^], a mark indicating an insertion, and “carrot,” an orange root vegetable preferred by bunnies. We’re not talking about them here.)
“Carat” and “karat” are, technically, both accurate for the purity of gold, though only “carat” can be used for gems. M-W lists “carat” as a variant form of “karat” and says, “The substitution of karat for carat in regard to precious stones is considered incorrect, whereas the reverse—using carat in place of karat to indicate the pureness or fineness of gold—is considered acceptable. (English strikes again.)”
It is nearly impossible, however, to find “carat” referring to “karat” IRL, at least in American publications. In Nexis, “carat gold” appears almost universally in British, South Asian, and South African publications. The only two uses of “carat” for gold we could find in Nexis in the past five years in American publications were both in Tennessee newspapers, one in a report about the theft of a safe, and one in a report about the theft of jewelry and other items. Since both were taken from police reports, it’s unclear who decided on that spelling.
“Carat” is the one used most frequently in books, according to this Google ngram, not surprising given that the British world uses “carat” for both measurements.
In the United States, keeping them separate makes life easier for most journalists, especially since they measure not just different substances but use different metrics: A “carat” measures weight (mass) and a “karat” measures purity. Except, as the science site says, “karat” measures mass too:
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.
Don’t know about you, but we’ll stick with the “1/24th” measurement instead of mass Otherwise, it’s a mess.