The 4 c's of Diamonds - Part 4 of 5 - Carat

Measuring Diamond Weight in Carats

GIA (Gemological Institute of America), the leading authority in diamond evaluation, includes Carat weight in the 4Cs (the other three being Cut, Clarity, and Colour) as one of the main factors that define the value of a diamond. One carat weighs 1/5 of a gram (or 200 mg), which is subdivided into 100 points. When a jeweller places a diamond on an electronic scale, the number he pays attention to is calculated up to two decimal points, despite the fact that such devices are sensitive enough to go beyond this precision. And for those who might be a bit confused, diamond carats are different from the ones that measure the purity in gold alloys and are also written in English as ‘carat’.

Below is a carat to mm size chart for round, brilliant cut Diamonds

Once Upon a Time

In ancient times, and especially along the Southeastern shores of the Mediterranean, people measured gems against the carob bean or seed. They actually used its uniform weight as a convention. The word carat appears first in English texts in the mid-15th century and comes from the Italian ‘carato’ with roots in the Greek word ‘keration’ that translates into carob seed. Back then, each country seemed to have its own carat, which would double as gem and gold weight measurement. Another ancient equivalent to the carat was four grains of rice and that’s why a quarter of a carat was also called a ‘one grainer’.

Below - 0.25ct round brilliant cut crossover engagement ring

Until Today

In the 19th century the need to standardize globally the metric unit for diamond weight was apparent, so an ‘international carat’ of 205mg was agreed in 1877 by the Syndical Chamber of Diamond Merchants in Paris, where it was rounded to 200mg in 1907 by the fourth General Conference of the Metric Convention. It soon became law in France and was gradually adopted all over the world.  

Below - 0.80ct round brilliant cut 6 claw engagement ring


A diamond’s weight is closely related to its measurements, mainly because a nice value-for-money diamond has to look its weight. There is a narrow range of suggested measurements for a round brilliant cut diamond at each weight. The classic case of the one-carat diamond comes with a suggested diameter of 6.4-6.5 mm. A smaller diameter for a given carat weight means lower quality and lower price. A larger diameter – even for fractions of a millimeter – might make your stone look bigger than you expect, but with less of a brilliance or ‘fire’. It affects the way this stone reflects light and increases the possibility of light leakage from the sides or the bottom (culet), which reduces the desirability of the stone. For a 1.50ct diamond the diameter should be around 7.5mm, for a 2.00ct it should be 8.1-8.2mm, and you will see a few more such measurements in the following carat-mm chart. As a rule of thumb, you should avoid round diamonds that are cut either too shallow or too deep. Jewellers who want to save up to 25% more weight of the raw stone they are cutting from, design in fancy shapes (not round) such as pear, Marquise or oval ones that often mask some of the imperfections.

Below - 1.00ct round brilliant cut north, south, east, west 4 claw engagement ring


Diamonds of great size are a symbol of rarity and power. In fact, as the carat weight gets larger, the value of the diamond increases disproportionately to reach great heights. For diamond insiders, that applies to super tiny diamonds too, which are also in great demand by jewellers. Our international guide to pricing is the Rapaport report, which is based on the evaluation of the 4Cs for each diamond. Along with a GIA report which is the gold standard in evaluating diamonds, we make sure that we pick diamonds of outmost desirability (hand-picked to reflect the light well and to look their best at each pricing bracket). Beyond technical characteristics, each stone is unique and it has to be the most beautiful one, every other characteristic being equal. 

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