Both words on one pair of studs. Carats of diamond, karats of gold. 18 karat gold studs.
Carat with a C weighs gemstones: one carat is exactly 0.2 grams. Karat with a K grades gold purity: 24 karat is pure, 18 karat is 75% gold. Two different measurements, two different letters, one endless source of confusion, not helped by the fact that much of the world spells both with a C. Here is the whole tangle, untangled.
Carat: the weight of a stone
The gem carat is a mass unit, standardised at 0.2 grams, so a 5 carat stone weighs one gram. The word descends from the carob seed, once used as a counterweight on gem scales because the seeds were conveniently uniform. Points subdivide it: 100 points to the carat, so a 25 point diamond is 0.25 ct. Two honest notes buyers need: carat weight is not size, since cut proportions decide how big a stone looks face-up, an antique rose cut spreads far wider than a modern brilliant of the same weight; and "total carat weight" (tcw) sums every stone in a piece, so a 1 ct tcw cluster ring contains many small stones, not one impressive one.
Karat: the purity of gold
The gold karat divides purity into 24ths: 24K is pure gold, 18K is 18/24 = 75%, 14K is 58.5%, 9K is 37.5%. Continental Europe stamps the same facts millesimally, 750, 585, 375, which is why a Danish ring says 585 where an American one says 14K, identical claims in different dialects, all covered in the hallmark guide. Higher karat means more gold, richer colour and softer metal, the trade-offs weighed in 14K vs 18K.
Why the two get confused
| Carat (C) | Karat (K) | |
|---|---|---|
| Measures | Gemstone weight | Gold purity |
| Unit | 0.2 grams | 24ths of pure gold |
| Written | ct, e.g. 1.50 ct | K or kt, e.g. 18K; or 750 millesimally |
| Applies to | Diamonds and gems | Gold only |
| British spelling | carat | also carat, context decides |
The last row is the real culprit: British and much international usage spells both "carat", so an English listing saying "18 carat gold with a 2 carat sapphire" is using one word for two measurements. The K spelling is an American convenience that stuck for gold. Old European paperwork adds a third cousin: the metric carat only standardised internationally in the early twentieth century, so very old stone weights can differ slightly from modern recalculations.
Reading listings without being fooled
The traps are predictable. "24K gold plated" is a purity claim about a microscopic skin, the plating games covered in the real-gold guide. High carat numbers attached to obviously modest stones usually mean tcw across many chips. Stone weights in old rings are almost always estimates unless the stone has been unset and weighed, honest sellers say "estimated" and show the maths from measurements. And gold weight in grams is a different number from both: a listing stating grams, karat and stone carats separately, as mine do, is giving you all three measurements without the pun.
Frequently asked questions
Is carat the same as karat?
No: carat weighs gemstones (0.2 g each), karat grades gold purity (24ths). British English spells both "carat" and lets context decide, which is where the confusion lives.
How much does a 1 carat diamond weigh?
Exactly 0.2 grams. Five carats make a gram, and 100 points make a carat, so a 50 point stone is 0.50 ct.
What does 18 karat mean?
18 parts gold in 24, so 75% pure gold, stamped 750 in the European millesimal system. The metal facts behind the stamp are in the hallmark guide.
Is higher carat weight always a bigger-looking stone?
No; cut decides face-up size. Antique cuts often look larger per carat than modern brilliants, and deep-cut stones hide weight underneath.
What does tcw mean?
Total carat weight: the sum of all stones in a piece. A 1 ct tcw cluster is many small stones; a 1 ct solitaire is one. The price difference is substantial.
Why do old documents give odd stone weights?
The metric carat of 0.2 g was only standardised internationally in the early 1900s; earlier local carats varied slightly, so antique paperwork can disagree with a modern scale by design, not deceit.
Read next: 14K vs 18K gold · How much is my gold worth? · What is a cluster ring?
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