The beam of the balance sways and slowly settles, because carob seeds fall one after another onto the right-hand pan — the very seeds the unit owes its name to. In the middle lies a ruby cut as a round brilliant: glints run across its facets, and a white beam enters on the left and leaves on the right as a fan of four colours, because the stone splits light as well as a prism does. Below, the slider of a calliper travels along the scale and stops exactly at six and a half millimetres — the diameter at which a brilliant weighs one carat.
Unit passport
Non-SI · quantity: mass sheet 1/1 · rev. 2026-09Carat ct
Non-SI unit · quantity: mass · exactly 200 mg
The seed that turned out not to be so uniform
For centuries Mediterranean traders balanced stones against carob seeds and passed down the belief that the seeds are remarkably constant in weight and can therefore stand in for weights. The pretty explanation held until two thousand and six, when the seeds were finally just weighed by the thousand: the spread proved perfectly ordinary, about a quarter of the mean, no better than peas or beans.
So it was habit, not nature, that did the work: a handful of seeds averages itself out, and a trader who counted out a score of them was off by little, while each single seed lied shamelessly. The unit rested on agreement between the parties, not on the precision of the grain, and that is exactly why it eventually had to be fixed as a number.
The number chosen was a round one: two hundred milligrams, a fifth of a gram. It is slightly less than the old city carats traded in London, Paris and Amsterdam, but it is the same everywhere, and since nineteen hundred and seven a stone flying from one country to another no longer changes its weight at the border.
The symbol is written as two lower-case Latin letters without a full stop and separated from the number by a space: 1.25 ct. National abbreviations are acceptable in running text, but passports and certificates use the international one, since a stone rarely stays in the country where it was weighed.
The carat of mass and the carat of gold are different things with a common ancestor: the first weighs two hundred milligrams, the second is one twenty-fourth part of an alloy's fineness and weighs nothing at all. American English tells them apart by spelling, carat and karat; elsewhere one has to look at what is being discussed: for a stone the carat is weight, for the setting it is fineness.
01 · Definition
two hundred milligrams · density · price stepsThe metric carat is a mass of two hundred milligrams, fixed by agreement and not derived from any constant of nature. A hundredth of a carat is called a point and weighs two milligrams, so a stone's weight on a certificate is always written with two decimals: 0.73 ct is read as seventy-three points.
It is worth remembering that the carat measures mass while the buyer looks at size, and between the two stands density. Corundum, of which ruby and sapphire are made, is about a seventh heavier than diamond, while beryl, that is emerald, is lighter by almost a quarter, so three stones of the same diameter come out quite different on the scales.
Hence the rule a jeweller repeats every day: a carat is not a size. A one-carat sapphire is noticeably smaller than a one-carat emerald, and a buyer used to diamonds is surprised every time to receive in the palm a stone smaller than expected.
There is also a reverse trick, spoken of less often. A cutter who ends up with a heavy crystal is free to leave the pavilion deeper than it should be: the diameter does not grow, but the weight does, and the stone sells at the price of a large one while looking small. This hidden weight shows only in the measurements, which is why a decent certificate always lists three dimensions in millimetres next to the carats.
Why stones of the same size weigh differently
On the left the stone is drawn at natural size, from above and from the side, on a millimetre grid: the diameter slider widens it, the depth slider deepens the pavilion, and together with density the two give the mass. On the right the same mass turns into money, and the main thing becomes visible: the price rises not smoothly but in steps, because the market pays for round numbers. The grey line shows what the stone would cost if the carat were merely a measure of weight and not a reason to haggle; the gap between it and the blue ladder is the premium for a round weight.
02 · Conversion
mass · stone size · old caratsOne weight in three conversations
The first tab spreads the mass you enter across units, from milligrams to the troy ounce. The second answers the question the buyer asks: what does such a stone look like if it is a diamond, a corundum or a beryl. The third weighs the same stone in the city carats traded before the reform.
A rule to remember: five carats make exactly one gram, and a round one-carat brilliant is six and a half millimetres across — about the size of a large pea.
The total weight of a piece of jewellery and the weight of its main stone are different lines. The words “diamonds 1.00 ct” may mean one stone or fifty chips in a pavé row, and the two cases differ in price some thirty times, even though the scales show the same figure.
| Quantity | Value | Note |
|---|---|---|
| {k} | {v} | {note} |
{text}
04 · Measuring instruments
balance · micrometer · sieves · hydrostaticsFour ways to find how many carats a stone holds
A balance that fears breathing
A stone is weighed with a resolution of a thousandth of a carat, that is two ten-thousandths of a gram, and at that level everything interferes: a draught from the door, the warmth of a palm, a speck of dust on the pan. So the balance stands in a glass cabinet on a heavy table, the stone is left to settle to room temperature, and the weights are kept as a separate verified set, because the cost of a one-point error on a large stone easily exceeds the price of the balance itself.
When the stone cannot be taken out of the setting
A set stone cannot be put on the scales, yet its weight is needed both for appraisal and for insurance. Then a spring gauge with jaws is used to measure the diameter and depth right in the piece, and the mass is calculated from the cutting formula with a correction for the girdle. The accuracy is about three to five per cent, and a certificate records such a weight on a separate line with the word “estimated” — the difference between a measured and a calculated weight matters here, because the second is always somewhat debatable.
Stones counted with a sieve
Weighing chips of two or three points one by one makes no sense: it would take a day, and the whole handful is worth little. Such small stones are passed through a set of sieves with calibrated holes, and each sieve holds back its own size group, which is at once assigned a range of weight. The group is then weighed as a whole and divided by the number of stones, so an invoice lists for small stones not the weight of a stone but the weight of the parcel and the average size — a custom that makes comparing jewellery by the total weight of its stones almost pointless.
A balance that reveals not weight but species
The same stone is weighed twice, in air and suspended in water, and the difference between the readings gives the density, and every species has its own: three and a half for diamond, exactly four for corundum, two point seven for beryl. The method is older than any optics and is still irreplaceable, because it tells apart stones that look alike under a loupe — for instance diamond and its cubic zirconia double, which is almost twice as heavy and so gives itself away before it has a chance to sparkle.
05 · Writing rules
two decimals · rounding downHundredths that must not be rounded up
The rules for handling the carat are almost entirely devoted to one thing: not letting a stone pass for larger than it is. Hence the notation in hundredths, rounding down, and the obligatory note on whether the stone was weighed or calculated from its dimensions.
The second line on the right is not pedantry but the industry's most lucrative error: crossing a round weight raises a stone's price by a quarter, so rounding up by four thousandths of a carat is worth more than the whole rest of the parcel. The third line says the same from the other side: the word “nearly” in a weight description means nothing and is forbidden on certificates. The sixth looks like a trifle, but it is precisely the zeros after the decimal point that show a stone was weighed to the point and not named by eye.
06 · Neighbouring units
grain · ounce · finenessAround the carat crowd units it is confused with daily: the pearl grain, a quarter of a carat; the troy ounce, which holds over a hundred and fifty carats; and the karat of gold, which has nothing to do with mass at all.
A quarter of a carat, the unit of the pearl trade. Pearls are not cut, but they are weighed the same way, and old inventories of strands are still kept in grains.
The measure of precious metals: it holds exactly 155.517 carats. The stone is counted in carats, the setting in ounces, and the bill for a single piece comes in two different systems.
The share of pure gold in an alloy: eighteen karats means eighteen parts out of twenty-four, that is 750 fineness. The quantity is dimensionless, yet shares its name with a measure of weight.
07 · Historical section
archive · from seed to agreementHow every city had its own carat
{title}
{text}
While every city kept its own carat, a large stone sold in Vienna and reweighed in Florence gained almost five per cent in weight, and disputes about it were a routine matter of merchant correspondence.
The legend of the constant seed
Tradition has it that carob seeds all weigh the same, so an ancient trader could put them on the pan instead of weights without risk of being cheated. The legend is pretty, lives in every catalogue, and resisted checking simply because it never occurred to anyone to check it.
In two thousand and six biologists finally weighed several thousand seeds gathered from Mediterranean trees and found the most ordinary spread: about a quarter of the mean weight, just as much as in the seeds of other legumes. The mean did indeed turn out close to two tenths of a gram, but a single seed could be half as heavy as its neighbour.
The answer is that a trader never used a single seed. He took a handful, and a handful averages, so the counting error fell in proportion to the square root of the number of grains; with a score of seeds it was already no more than a few per cent, which was acceptable for a deal. The precision was created not by nature but by the arithmetic of large numbers, which the parties to the deal knew nothing about.
The story is instructive beyond the jewellery trade too: a unit lived for two millennia on an explanation that turned out to be wrong, and suffered not at all from it, since it rested on agreement between the parties. When the agreement had to be written as a number, the seed was quietly sent off to the etymological dictionary, and the carat became exactly two hundred milligrams.
Catalogue of quantities
mass, fineness and gemsSI base units
kilogram and metre highlighted — the weight and measurements of a stoneStones by weight
grams and diameter calculated from carats| Stone | Carat | Gram | Ø of a round stone, mm | What is notable |
|---|---|---|---|---|
| {name} | {ct} | {g} | {d} | {note} |
Note the last column: mass grows as the cube of the diameter, so a stone twice as wide weighs eight times as much. It is this cubic dependence that makes large stones so rare — to double the diameter, nature has to find eight times more matter in a single crystal without a single crack.