Why gold is measured in karats, and why 24 is pure.

Karats are 24ths

Hallmark numbers are parts per thousand, and they're minimums: 14K is marked 585, a little above the exact 583.3, so a piece meets the standard with room to spare. See the gold karat chart.

Where the 24 comes from

The word comes, by way of Italian carato and Arabic qīrāṭ, from the Greek keration, the seed of the carob tree, which was used as a small weight. Around 309 to 312 AD, the emperor Constantine introduced a gold coin, the solidus, struck at 72 to the Roman pound. Each solidus weighed 24 siliquae, the Latin for carob seeds. The usual explanation is that 24 parts of a gold coin became the way to describe gold's purity: a coin that was all gold was 24 parts, and an alloy was so many parts gold out of 24.

A myth to drop: carob seeds are often said to be remarkably uniform in weight, which is why they were used. A 2006 study measured them and found they vary about as much as the seeds of other plants, and people can see a weight difference of about 5% by eye. The seeds were used, but not because they were uniform.

Is 24K really pure?

Pure gold is 24K, and bullion is usually 999 or 9999 fine. U.S. rules say an item can be called simply "gold" only if it's "fine (24 karat) gold"; anything less must state its karat, like 14K. U.S. law lets a marked karat be only slightly under the stated fineness, so a 24K mark means very close to pure gold.

Gold is at $3,925 an ounce (the Kitco bid). For what each karat is worth per gram, see scrap gold per gram.

Sources

• Merriam-Webster, carat and karat.
• Turnbull and others, "Seed size variability: from carob to carats", Biology Letters, 2006.
• University of Warsaw, solidus.
• 16 C.F.R. §23.3 (FTC guides on gold); 15 U.S.C. §295; U.K. Hallmarking Act 1973, Schedule 2.

Gold guides: Karat vs carat · Gold karat chart · Gold stamps · 24K gold · Troy ounce vs ounce · All guides

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