The Carat Was Never a Carob Seed

The carat carob seed myth holds that carats are named after carob seeds because every carob seed weighs the same. They don't. A 2006 study in Biology Letters found carob seeds vary in weight by about 23 per cent, close to the 25 per cent average across 63 species. The name is real. The uniformity never was.

A carat is a unit of mass equal to exactly 200 milligrams — one fifth of a gram — used for weighing gemstones and pearls. It divides into 100 points, so one point is two milligrams. That precision is barely a century old, and no seed had anything to do with fixing it.

The etymology is sound, though. Carat first appears in English in the mid-fifteenth century, arriving from Italian carato, which came through Arabic qirat from the Greek kerátion — a diminutive of keras, "horn", and the Greek name for the carob seed. Roman weights ran on the siliqua, Latin for the carob pod, at one 1,728th of a pound. So a carob really is in there. What gets repeated wrongly is why.

Do carob seeds really all weigh the same?

No. In September 2006, Lindsay Turnbull, Luis Santamaría, Toni Martorell, Joan Rallo and Andy Hector published "Seed size variability: from carob to carats" in Biology Letters (volume 2, issue 3, pages 397–400). They weighed carob seeds and set the spread against published figures for other plants. Their carob sample had a coefficient of variation of roughly 23 per cent. The average across 63 species in the literature was 25 per cent. Statistically, carob is an unremarkable seed.

One detail in that paper makes the legend feel almost plausible. Seeds from female carob trees averaged 200.5 milligrams, landing by coincidence within half a milligram of the modern carat. Seeds from hermaphrodite trees came in lighter, at 175.8 milligrams, which the authors put down to the metabolic cost of making pollen. The mean is in the right neighbourhood. The scatter around it is not.

Put it in working terms. Tip a handful of unsorted carob seeds onto a balance, use them one at a time as counterweights, and your "one carat" wanders by tens of milligrams. That is the difference between a stone you would happily buy and one you would argue about.

Where did the carat carob seed myth come from?

The same paper offers an answer, and it is better than the myth. The researchers ran a perception test: could people pick the heavier of two carob seeds by eye? They could, reliably, once the difference reached around five per cent. A merchant sorting by sight could therefore throw out the runts and the giants and keep a working set far tighter than the tree supplied.

Then the good part. The old pre-metric carat standards varied by roughly that same five per cent. The precision of the historic carat matches the precision of the human eye, not the precision of the plant. Uniformity was manufactured, seed by seed, on somebody's bench. Somewhere along the way the credit passed from the sorter to the tree.

What did a carat weigh before 1907?

It depended entirely on where you were standing. Tao Zhengzhang surveyed the old standards in the Chinese Journal of Geochemistry in 1991, and the range is wide — from about 187 milligrams in Cyprus to nearly 216 milligrams in Livorno.

Figures as compiled by Tao Zhengzhang, 1991; the percentages are ours. Look at Florence and Livorno. Under a hundred kilometres apart, both in Tuscany, and their carats differed by about nine and a half per cent. Buy a parcel by weight in one, sell it by weight in the other, and the arithmetic moves without a single stone changing hands honestly or otherwise. That was the actual problem. Not romance. Reconciliation.

How did the carat become exactly 200 milligrams?

Slowly, and by committee. In 1871 the Syndical Chamber of Jewellers in Paris proposed an "international carat" of 205 milligrams, and the Syndical Chamber of Diamond Merchants in Paris accepted it in 1877. It was a round number close to the northern European standards, and it never took hold everywhere.

The metric carat followed. The International Committee for Weights and Measures put forward 200 milligrams, exactly one fifth of a gram, and the Fourth General Conference on Weights and Measures — meeting in Paris from 15 to 22 October 1907 — accepted it unanimously. France made it compulsory by law soon after. England moved at its own pace, since metric units had already been permitted there under the Weights and Measures (Metric System) Act of 1897. The United States adopted the metric carat on 1 July 1913 and the United Kingdom in the spring of 1914, though sources disagree on the exact day.

Notice what the committee picked. Not the average of the old carats. Not the London standard, which carried the most commercial weight. They chose the number that divides cleanly into the gram, because a unit that plugs into the rest of the metric system is worth more than a unit that flatters whoever happens to be biggest. London gave up about 2.6 per cent, Amsterdam about 2.8. Everyone gained arithmetic.

Why a tidy myth outlives the thing it explains

The carob story survives because it does a job the true story cannot. It explains an odd word with a picture — a trader, a brass balance, a small brown seed — and then it stops. No dates, no committees, no coefficients of variation. You can repeat it at dinner without checking anything.

The accurate version needs a botanical statistic, a perception experiment and a conference in Paris. It is more interesting and much harder to carry. Twenty years on from the Biology Letters paper, the seed story still turns up on jewellery websites and in gemmology primers. Corrections travel badly. They always have.

There is a quiet irony in it. The myth credits nature with a consistency that people had to build by hand, and it does so in the one unit that eventually had to be fixed by international agreement precisely because it was not consistent.

What did standardisation actually buy the trade?

A fixed carat turned gemstones into something quotable over a wire. A 1.02 carat stone means the same mass in Antwerp, Mumbai and New York, so price per carat became a usable number and parcels could move between people who had never met. Much of the modern trade — the grading reports, the price lists, the ability to buy a loose stone you have never held — rests on that 1907 agreement.

A quirk came with it. Because weight is now exact and publicly stated, the market prices in steps, and stones sitting just under a round weight generally sell for less per carat than stones just over it. A 0.98 carat stone is not visibly smaller than a 1.00. It is priced as though it were. That is a consequence of standardisation rather than of geology, and it is worth knowing before you insist on a round number.

Does a higher carat weight always look bigger?

Not reliably, and this is where the old confusion still costs people money. Carat measures mass, not spread. Two round brilliants of identical carat weight can differ in diameter depending on how deep they are cut, because a deep stone hides weight in its belly where nobody will ever see it. Shape matters as well — elongated cuts such as ovals and marquises tend to read larger face-up than a round of the same weight.

So read the weight and the measurements together. A grading report states carat weight to two decimal places alongside the stone's dimensions in millimetres, and those two lines tell you different things. Our engagement ring guide covers how weight trades against cut when the budget is fixed, and what IGI certification actually confirms sets out what a report does and does not promise. To see the effect for yourself, compare a few solitaire rings at the same carat weight in different cuts.

One last piece of housekeeping, since both words trace back to the same carob root. Carat is the weight of a stone. Karat is the purity of gold, and the difference between 24K, 18K, 14K and 9K has nothing to do with the diamond sitting on top of it.

We think about this more than is strictly reasonable, because every stone we set gets weighed and every weight gets written down. Our diamonds are lab-grown and IGI-certified, with the report number laser-inscribed on the girdle, so the figure on the paper and the figure on the stone are the same figure. Each piece is made to order at our own bench in Surat, in solid 9 to 18 carat gold, never plated. If you would like someone to read a report with you before you commit to anything, write to us. We would rather talk you through the numbers than sell you a round one.