Carat: what the number on a diamond actually weighs
A single listing in my shop gives the weight of its stones to 2 decimal places. The Tennis Chain Diamond Necklace carries brilliant-cut GH SI1 to SI2 diamonds with a total weight of 2.70 carats. That figure is not a size and it is not an opinion. A carat is a unit of weight, and of the 4 figures a diamond is graded on it is the only figure read off a scale rather than decided by a person looking down a loupe. Writing in Professional Jeweller in 2021, the qualified diamond grader Robert Robins made the point plainly: carat is a measurement of weight rather than human perception, so it cannot easily be manipulated.
I have made jewellery by hand in London for 22 years, from a stall on Portobello Road to counters at Liberty, Selfridges, Harvey Nichols and Harrods. I have weighed a great many stones in that time, and had a great many conversations go sideways because 2 different numbers are said the same way.
What a carat actually weighs
A metric carat is 0.2 grams, which is 200 milligrams. It divides into 100 points, so a 0.25ct stone is 25 points at the bench and a 0.50ct stone is 50. Nothing in that number tells you how wide the stone looks from above or how it behaves in daylight. It tells you what it weighs.
A total weight is not 1 stone
The 2.70 carats on that necklace is every diamond in the row added together, not the weight of any single stone in it. The same goes for a pavé setting, a halo, or a line of stones down a bracelet, and it is the most common place a figure gets read as something grander than it is.
10 small stones adding up to a carat and 1 single carat stone will never carry the same price, because a large clean stone is far rarer. So when you read a total carat weight, the useful next question is how many stones it is spread across.
Carat is not the same number as karat
Carat with a c is the weight of a stone. Karat with a k, the number in 9kt or 14kt, is the purity of the gold, counted in 24ths, so 14kt is 14 parts gold in 24. Carat is printed on a report. Karat is stamped into the metal, and the hallmark inside a piece is where you read it. A 14kt solid gold setting and a 0.25ct diamond are 2 unrelated facts about the same earring.
Why the weight is the number worth trusting
Colour and clarity are grades awarded by eye, and in the same column Robins describes a dealer answering an order by asking whether he wanted a good SI2 or a bad SI2. 2 stones can hold the same clarity grade and look nothing alike. Weight does not move like that. It is the line on a report you could check yourself with a decent set of scales.
The piece that says all of this on its own page is the Tennis Chain Diamond Necklace, £12,000 in 14kt solid yellow or 14kt solid white gold, 16.5in round and 3mm wide. Its 2.70 carats sit in a continuous line of small brilliant cuts, and the listing names the colour and the clarity alongside the weight.
What I notice is that people apologise before asking what a carat is, as though it were something they were supposed to already know. It is a unit of weight that happens to be very small. Knowing it is 0.2 grams is the whole of it.
The weight you already own
Weight is also why an inherited stone is worth asking about. A setting wears, a claw lifts, a design stops being worn, but the carat weight of a stone does not change when the piece around it does. If you have an old cut stone in a drawer, or a ring you have not put on in 10 years, that weight is already yours. Second Life is my remodelling service: send me a photograph and I will tell you what is there and what it could become. The estimate is free and there is no obligation. For a remade piece to be ready for Christmas I need the photographs by Sunday 1 November.
Lower down, the Diamond Huggie Hoops are £70 in sterling silver and £550 in 14kt solid white gold, and the listing says outright that the silver pair uses cubic zirconia and the solid gold pair 0.25ct genuine diamonds. The silver I work with is recycled, and there are more of these in earrings.
The Spinning Infinity Necklace is £85 in sterling silver, £100 in the vermeils and £950 in 14kt solid gold, with 0.8ct of stones set across the symbol. And if the stone is not the point at all, an engraved Initial Disc Charm is £25 in sterling silver and £40 in gold or rose gold vermeil, 1 initial per disc.
Everything set with a stone sits in the diamonds collection, and the pieces where karat is the number that matters are in 9kt solid gold.
The second life is the real one. Anna Lou x.
If you are holding a stone and do not know what you have, send me a photograph at anna@annalouoflondon.com or on WhatsApp.
Questions I get asked
How much does 1 carat weigh?
A metric carat is 0.2 grams, or 200 milligrams, and it divides into 100 points. So a 0.50ct stone is 50 points and a 0.25ct stone, the weight in the solid gold Diamond Huggie Hoops, is 25.
Does a higher carat weight always mean a bigger looking stone?
No. 2 stones of identical weight can look different sizes from above, because weight can sit in the depth of a stone instead of across its face. A shallow cut stone can read wider than a deeper stone of the same weight, so ask about the measurement across the top as well as the carat weight.
Is the karat in 14kt gold the same as the carat in a diamond?
No. They are 2 different measurements that sound alike. Carat is the weight of a stone. Karat is how much of the metal is gold, counted in 24ths, so 9kt is 9 parts in 24 and 14kt is 14. Everything in 9kt solid gold is solid metal all the way through rather than plated.