So you and your significant other have been dating for a while, and you’ve finally decided to pop the question. Well the first thing you’re going to need is a ring, right? Preferably diamond, and costing the equivalent of three months’ salary? After all it’s traditional, isn’t it? Never mind that this so-called ‘tradition’ is less than 100 years old, invented by diamond giant DeBeers in the 1930s to sell more surprisingly common diamonds – that’s good enough for you, damn it! But now comes the tricky part: what cut of diamond to buy? Oval? Pear? Cushion? Heart? Princess? And what mounting? And how many carats? And what even is a carat anyway? And why is it used to measure both gold and jewels and not really anything else? How did this come to be?

To begin with, there are actually two different ‘carats’ used in the jewelry trade – one spelled with a ‘c’ and the other with a ‘k’ – though mostly in the United States. The carat with a ‘c’ is a unit of mass used for measuring gemstones and pearls, and is equivalent to 200 milligrams, 0.00705 standard or avoirdupois [“av-wahr-doo-pwah”] ounces, or 0.00643 troy ounces. One carat is divisible into 100 points of 2 milligrams each. And in case you’re wondering what the heck a “troy ounce” is, this is another unit of mass peculiar to the precious metals trade, and is equivalent 31.103 grams. One troy ounce can be divided into 20 pennyweights, which in turn can be divided into 24 grains, while 12 troy ounces make up one troy pound. The troy grain, originally based on the average weight of a grain of barley, is identical to the avoirdupois grain and is equivalent to 64.799 milligrams. Thus, one carat is equivalent to 3.086 grains. Confused yet? Well, just be thankful for the metric system…

The term “carat” comes to us from the Italian carato, which itself is derived from the Arabic qirat and the Ancient Greek keráton, meaning “carob seed.” The carob, or Ceratonia siliqua, is an evergreen tree native to the Mediterranean which produces long, hard brown seed pods. These pods were thought by the Greeks to resemble goat’s horns or keras – hence the name. The hard, round seeds found within these pods were long believed to be uniquely consistent in size and weight, and were thus used for thousands of years as standards for weighing gold and jewels. Today, however, it is known that carob seeds vary in mass just as much as other seeds, explaining why for much of history the value of the carat varied from region-to-region.

The carat as a unit for weighing diamonds first appeared in England in the 1570s, but its exact definition was not standardized until 1871, when the Paris-based Syndical Chamber of Jewellers proposed a fixed mass of 205 milligrams. This was officially accepted in 1877 and remained the global standard until 1907, when, at the General Conference of the Metric Convention, a new definition of 200 milligrams was accepted. This new definition was immediately made compulsory in France, but was slower to be adopted elsewhere. Today, however, the 200 milligram definition is accepted around the world. To date, the largest gem-quality gem diamond ever discovered is the Cullinan Diamond, excavated in 1905 at the Premier No.2 Mine in Cullinan, South Africa. Weighing in at a whopping 3,106 carats or 621.2 grams, the Cullinan was gifted in 1907 to King Edward VII of the United Kingdom, who had it cut unto a number of smaller gems. Two of these, Cullinan I and Cullinan II – AKA the First and Second Star of Africa – are part of the British Crown Jewels, being mounted in the Sovereign’s Sceptre with Cross and the Imperial State Crown, respectively. Meanwhile, the largest flawless diamond to be found and cut in a single piece is the 137.82 carat or 27.56 gram Paragon Diamond, mined in Brazil in the late 1990s and cut and set by London-based jeweller Graff Diamonds. Most diamond engagement rings, by contrast, are usually 1 carat or less, though the price of diamonds is not determined by mass alone; cut, colour, clarity, and brilliance are also key factors.

The second type of carat, often spelled with a ‘k’ in the United States, is also derived from the carob seed, but instead of mass measures the fineness or purity of precious metals – particularly gold, silver, and platinum. Fineness karats are measured and divided into 24 parts, with 24 karats indicating pure, unalloyed metal. Thus, if an alloy consists of 50% gold and 50% other metal, it will have a fineness of 12 karats. The 24-part system is thought to derive from the solidus, a standardized gold coin introduced by the Roman Emperor Constantine in the year 309 C.E. One solidus had a mass equivalent to 24 siliquae silver coins, whose mass was in turn based on the carob seed. Bullion coins and ingots are also sometimes referred to by their fine weight – that is, the weight of the pure precious metal within them. Thus, one troy ounce of 18-karat gold, which is composed of 75% gold, will have a fine weight of 0.75 troy ounces.

Adding to the confusion, jewellers also sometimes use a system called millesimal fineness, which rates the purity of precious metals by parts per thousand. In this system, “nines” are often used to denote exceptional purity, with 900 or 90.0% purity being “one nine fine”, 990 being “two nines fine” and 999 being “three nines fine.” The purest gold ever refined was produced by Australia’s Perth Mint in 1957 and was six nines fine – that is, 99.9999% pure – while the purest gold commonly refined today – produced by the Royal Canadian Mint – is five nines fine. For all practical purposes, however, any alloy above 995 fineness is considered essentially pure. Gold of this purity is rarely used outside of bullion ingots, collector coins, and gold plating because pure gold is very soft and easily deformed. This is, incidentally, why pirates in the movies bite gold coins; the softer the gold, the likelier it is to be pure. This method can also detect other forms of counterfeiting – such as coins made of lead or other base metals coated in a thin layer of gold.

For everyday use, gold is typically alloyed with other metals – particularly silver – with 14 and 18-karat gold being the most widely used for jewelry. Other metals can also be alloyed with gold to produce various colour effects. White gold, for instance, is alloyed with nickel, silver, or palladium; rose gold with copper; and green gold or electrum with silver or cadmium. Another commonly encountered alloy, Sterling Silver, is officially defined as containing 92.5% silver and 7.5% other metals – typically copper. However, most countries specify a minimum caratage – that is, the minimum amount of precious metal an alloy can contain and still be advertised as that metal. In the United States, for instance, the minimum caratage for gold is 10 karats or 41.6%; in France and the UK it is 9 karats or 37.5%; while in Denmark and Greece it is 8 karats or 33.3%.

And that, in a nutshell – or, in this case, a carob pod – is where the carat came from and how it is used in the world of jewels and precious metals.

Bonus Facts:

Have you ever wondered where the terms touchstone – that is, a test of quality – and hallmark – a characteristic feature – come from? Well, if you’ve been paying attention to the theme of this video, then you’ll probably have guessed that it has something to do with gold. Indeed, a touchstone is an actual piece of stone – typically black slate – which has been used for over 4,000 years to measure the purity of precious metals such as gold. To test a metal sample, it is wiped across the surface of the touchstone, leaving a characteristic streak. This streak can then be compared to that left by samples of known purity to estimate the purity of the metal. For greater accuracy, the streak can also be treated with chemicals such as Nitric Acid, which will attack lower but not higher-purity gold.

A hallmark, meanwhile, is a distinguishing mark stamped on a piece of precious metal by an assay office to certify its purity. The term derives from the Goldsmith’s Hall in London and was first recorded in 1721, while the generic meaning of “a distinguishing characteristic” first appeared in the 1860s. While hallmarks vary widely depending on country and manufacturer, for ease of international trade most countries accept a standard marking system known as the Common Control Mark or CCM, which consist of a set of scales for Gold, a diamond shape for Platinum, and a letter M for silver.