Diamond carat: your comprehensive guide

By Yves Lemay May 13, 20

“Carat” is the term used to refer to the weight of diamonds and other gemstones (not to be confused with the term “karat,” indicating the purity of gold). 


One carat is defined as one-fifth of a gram or 200 milligrams. 


A carat can further be divided into “points,” where one point is equal to 0.01 carat. 


While the size of a polished diamond is related to its carat weight, it is essential not to confuse carat weight with actual measurements, as there are other aspects of a diamond that can affect how large an individual stone appears—most importantly, the cut and shape of the stone. 


Proportions to which an individual diamond is cut, such as table and depth percentages, must be taken into account, as shallower stones can appear larger than deeper stones of the same weight.


Similarly, diamond shapes can have an impact, as elongated shapes tend to maximize carat weight, making them appear larger than rounded stones of the same weight. 


Furthermore, the appearance of a diamond’s size can also be affected by how the stone is set in a piece of jewelry, as well the size of the wearer’s finger (in the case of rings). 


Therefore it is essential to take all of these aspects into consideration when choosing a stone. 


History and background

The term carat originates from the Greek and Arabic names for the carob tree - Keration in Greek and Qirrat in Arabic. 


The dried seeds of the Carob (or Locust) tree were once widely used by trading merchants as counterweights for weighing gold, diamonds, gemstones and pearls due to their relatively consistent weight and size. 


The Byzantine era used glass pebbles, based on carob seeds, for weighing coins, which weighed in at 196 mg, consistent with the average weight of an individual carob seed.


dried seeds of the Carob (or Locust) yves lemay jewelry

However, their use eventually diminished as it was discovered that despite their visual uniformity, the seeds were not consistent in weight. 


Many attempts were made to standardize the measurement of gemstone weight, and it was only in 1907, at the Fourth General Conference on Weights and Measures, that the “carat” was adopted as the official metric measurement for gemstone weights.


In 1913 the United States officially accepted the ‘carat’ as the gemstone measurement, and in 1914 the United Kingdom and Europe followed suit. 


By the 1930s, the majority of the diamond and gemstone industry had agreed to the standardized measurement, which is still in use today.


Carat weight and size

Dimensions play an essential role in the appearance of a diamond. In addition to the carat weight, the distance across the top of the diamond must also be taken into consideration. 


A common misconception is that half a carat is half the size of one carat. 


A half-carat is half the weight of one carat, but the millimeter difference on a round stone is only 1.35mm. 


The average measurement for a 0.50ct stone is 5.00mm, while the average 1.00ct stone measures at 6.35mm. 


While carat weight may indicate a diamond’s size, the shape and cut of a stone also play a large part in determining how large or small the stone appears. 


An elongated shape such as the Marquise cut may appear larger than a rounded shape such as the round brilliant even if the two stones share the same weight. 


When comparing two stones of the same shape, however, it is essential to look at the cut grades and table and depth percentages, as shallower stones will tend to appear larger than deeper ones. 


Other aspects such as girdle thickness can also affect how large stone appears, while not necessarily affecting the quality of the stone.  


carat weight 0.5 scale of the diamond shapes yves lemay jewelry
carat weight 1 scale of the diamond shapes yves lemay jewelry
carat weight 2 scale of the diamond shapes yves lemay jewelry
carat weight 4 scale of the diamond shapes yves lemay jewelry

Here we can see the difference in size between different diamond shapes. 


We can also notice that even though 0.5 ct is eight times lighter than 4 ct, most of the various cuts in 0.5 ct are pretty much half the face-up appearance than a 4 ct diamond of the same shape.


Carat weight and price

Carat weight is one of the fundamental factors in determining the price of a diamond. 


As a general rule, the heavier the diamond, that is, the larger the carat weight, the more expensive it becomes. 


Price per carat is one of the best ways to compare the cost of similar diamonds. 


To calculate this, divide the cost of each stone by its carat weight.


Because they are scarcer, larger diamonds are in much higher demand than smaller stones and therefore command much higher prices per carat. 


A diamond that is double the size of another can be up to four times the price.


diamond price graph D IF according to carat weight yves lemay jewelry

This graph charts the relationship between diamond price* (in English Pounds) and the carat weight, as well as the price per carat for each weight. 


All the prices graphed are for GIA certified round brilliant cut diamonds of D color and IF clarity as on 31/01/2011. 


The blue line demonstrates the exponential increase typical of diamond pricing, proving that a doubling in carat size does not equate to a doubling in price since there is an increase in price per carat, as demonstrated by the red line.   


diamond price graph VS1 according to carat weight yves lemay jewelry

This graph charts the relationship between diamond price* (in English Pounds) and the carat weight, as well as the price per carat for each weight. 


All the prices graphed are for GIA certified round brilliant cut diamonds of G color and VS1 clarity as on 31/01/2011. 


The blue line demonstrates the exponential increase typical of diamond pricing, proving that a doubling in carat size does not equate to a doubling in price since there is an increase in price per carat, as demonstrated by the red line. 


Price/quality ratio sweet spot

-If you have a set budget, explore all your options, and you'll find that there is a wide range of diamond carat weights and qualities available in your price range. 


You can also have a look for the "magic weights." 


They are 1/2ct, 3/4ct, 90 points, 1ct, 1.5ct, 2ct, etc.


'Under-sizes' are diamonds that weigh just below a magic weight; they can be a bargain, but there are a lot less to choose from. 


- Also, keep in mind that the smaller the finger, the larger the diamond will appear. A 1½-carat diamond solitaire looks much larger on a size 4 finger than a size 8. 


- If you have already chosen a setting, make sure you select a diamond to fit. 


- Finally, if a large carat weight is vital to you, yet you're working within a budget, consider a diamond with a good cut, SI1–SI2 clarity, with an I or J color grade.


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