Know Your Carats: An Explanation on How Your Diamonds Are Weighed

In 2013, a 118-carat white diamond the size of a small egg sold for $30.6 million at an auction in Hong Kong. The stone, which broke world auction record, is by far the largest and most significant diamond ever graded by the Geological Institute of America (GIA).

Besides its flawless cut and clarity, what made this particular diamond so valuable was its carat weight.

So what is a carat, you might ask? And how does it affect a diamond’s price and value? Are size and carat the same thing? Read on to mine some interesting facts…

Why diamonds are weighed in carats

The weight of a diamond, as well as other gemstones, is expressed in carats. This practice originates from ancient times when gems are weighed against the carob beans, which are known for their uniform weight.

The term ‘carat’ is not to be mistaken with ‘karat’, which refers to the purity of gold.

Carat weight is typically expressed in points. One point is equal to a hundredth of a carat, so 50 points is half of a carat and 25 points is a quarter of a carter.

Besides the carob beans, rice grains were also used to weight diamonds. A carob bean is roughly the same in weight as 4 grains of rice. As such, a jeweller may refer to a 0.25 carat diamond as a one grainer – or a “twenty-five pointer.”

Diamond weights that exceed one carat are expressed in carats and decimals. For example, a 1.06 carat stone would be described as “one point oh six carats.”

Glass pebbles and other materials were also used as counterweights in the balance scale. However, pebbles of the same size weren’t exactly uniform in weight, so the use of such materials was later abolished. It wasn’t until 1907 when the Fourth General Conference on Weights and Measures standardized the metric carat (200mg) as the unit of weight for gemstones.

How diamonds are weighed

Weighing diamonds must be done with utmost precision, because a slight difference in carat weight can have large impact on price.

To get the exact measurement of carat weight, a piece of diamond is placed on precise electronic balances that are calibrated and verified on a daily basis according to ISO requirements.

Carat weight is first measured to a thousandth of a carat before it is rounded to a hundredth of a carat. For example, a carat weight of 0.709 is rounded up to 0.71 carats, while a 0.708 carat weight is rounded down to a 0.70 carats.

Carat weight and size

Some people use the terms ‘carat’ and ‘size’ interchangeably, but they are not necessarily the same thing.

A diamond’s size often has to do with its cut. The size of a full carat brilliant diamond can range from 6.20 mm to 6.80 mm in diameter, depending on the depth of the cut. Accordingly, a deep-cut diamond may have a narrower diameter and therefore may appear smaller than a shallow-cut stone of the same carat.

There are charts that would show you how big a diamond should look like relevant to its carat weight, but these are not 100 percent accurate. As carat weight increases, the apparent size becomes less predictable.

If you want to check the stone’s exact weight, have a look at its certification or grading report.

Why bigger diamonds are more valuable

Like most products, bigger items are often more expensive than smaller items of the same type or quality. But in the case of diamonds, the price difference is not simply due to the difference in size.

As carat weight gets larger, the value and price of the diamond increases disproportionately. So a diamond that is twice as large as an identical smaller stone may cost three or four times higher.

Why? Because pieces of rough stones are increasingly rare with size. In fact, over a million pieces of rough diamond must be mined to find a piece that’s big enough to produce a one carat stone.

Largest diamonds in the world

Over the course of history, diamond miners were able to unearth rough materials of truly remarkable size. The world’s largest uncut diamond, discovered in the Premier Mine in South Africa in 1905, weighs a staggering 3,106 carats or 1.33 pounds. It is dubbed as the “Cullinan” after mine owner Sir Thomas Cullinan.

The Cullinan was later cut into about 100 pieces in 1908, producing nine polished large diamonds and many smaller pieces. The largest piece cut from the rough is called “Cullinan I” or the “Star of Africa I”, which is mounted in the British Sovereign’s Royal Scepter. It weighs 530 carats and remained as the world’s largest cut, fine-quality colourless diamond until 1987, when the 545.67- carat diamond named “Golden Jubilee” stole its title.

Some of the largest diamonds ever discovered include “The Jonker” (726 carats), the “Spirit of de Grisogono” (312.24 carats), the “Incomparable”(407.48 carats) and the “Centenary”(273.85 carats).