You might have heard about the fact that the exact diamond proportions determine the cut of a diamond. If you have a stone with perfect diamond proportions, the diamond will refract the light in an excellent way. It will therefore exhibit a lot of sparkle, brilliance and scintillation. This is exactly what you are looking for in a beautiful diamond!

However, if the proportions of a diamond are poor, all the other 4C parameters can be as good as they like: The diamond will still look dull and lifeless!

So, what are the perfect diamond proportions?

To put it upfront, out of my own experience and research I think that these are the perfect diamond proportions for round cut diamonds:

In this post I will lay out to you why the exact proportions of a diamond have such a huge impact on the beauty of a diamond and why the above proportions will make sure that your diamond will exhibit an excellent light performance! I will be referring specifically to round cut diamonds as these are the ones that are most commonly bought.

What exactly are diamond proportions?

Diamond proportions refer to specific parameters within a diamond such as the angles to which it is cut and the exact length of different facets. Length measurements such as table size, pavilion depth and crown height are measured in % relative to a diamond’s overall diameter. A diamond’s diameter is measured from one point of the girdle to the opposing point of the girdle.

So, if for instance the pavilion angle is not within the proposed 40.6° – 41.0° but instead is 42.0° it surely would not make any big difference?

Wrong!

A perfect cut diamond is proportioned in such a way that nearly all the light that enters the diamond gets returned to the viewer’s eye. A seemingly minute change in the pavilion or crown angle can make a big difference to the sparkle, fire and scintillation of a diamond. The same applies to the crucial parameters such as the table size and the depth.

One of the first pioneers to come up with a reasonable idea of what kind of exact proportions a diamond would need to have for an excellent light performance was Marcel Tolkowsky in 1919. He was a renowned gemmologist and mathematician and published the so-called Tolkowsky Ideal cut in his thesis with the title “Diamond Design, A Study of the Reflection and Refraction of Light in a Diamond!”

A diamond that has the exact proportions of the Tolkowsky Ideal Cut will invariably be a beautiful diamond and this even applies nearly 100 years after the discovery of the Tolkowsky Ideal Cut. However, very reputable grading laboratories such as GIA and AGS conducted a ton of research and found out that there is a specific range of many different proportions that a diamond with an excellent light performance can have!

This is also due to the fact that there is always a tradeoff between brilliance and fire within a diamond. This tradeoff can vary depending on the exact measurements of crown and pavilion angles. The brilliance of a diamond refers to the reflections of white light being returned to the viewer’s eyes. Fire on the other hand refers to the rainbow colors that can be seen by the viewer as a result of light dispersion.

Thus, the more brilliance a diamond has, the less fire it tends to exhibit and vice versa. Therefore, in general it is desirable to strike a balance between these two factors.

What should the perfect relationship between crown angles and pavilion angles be like?

Now, going even deeper there is even a perfect balance that you can strike between crown and pavilion angles to create a really exceptional light performance in a diamond.

As you might have heard about, it is especially the pavilion angles  that play a crucial role in determining how much light gets returned to the viewer’s eye. For instance a too shallow pavilion angle will result in a shallow diamond. Likewise a too steep pavilion angle will result in a deep diamond:

Within a certain degree however a slightly too shallow or slightly too steep pavilion angle can be counterbalanced by an appropriate crown angle.

As demonstrated on the table at the top researchers have found that a pavilion angle within the range of 40.6° and 41.0° gives a diamond the most amount of brilliance.

Thus, a slightly too shallow pavilion angle of 40.5° has ideally to be compensated for by a steeper crown angle of for instance 35°. The same applies to a diamond with a slightly steep pavilion angle of 41° which would be ideally counterbalanced by a 34° crown angle.

If you can remember to pay attention to these details you can be sure to get a diamond with exceptional light performance!

Why should a diamond be within these perfect proportions?

The above mentioned diamond proportions are the perfect proportions for round cut diamonds for several reasons.

For one thing, the suggested diamond proportions are even stricter than the proportions demanded by GIA or AGS for the best cut grade! Thus, any diamond that is within these suggested proportions will be much more likely than a diamond outside these proportions to exhibit a very appealing light performance.

Furthermore, any diamond that is within these suggested proportions will get a Holloway Cut Advisor score below 2. I have explained in another post how the Holloway Cut Advisor works. It is a tool that takes most of the most important diamond proportions into consideration and evaluates the light performance. Basically, any diamond with a score below 2 is worth considering and any diamond with a score above 2 should be discarded.

So whenever you choose a diamond, try to make sure that it is within my suggested proportions. Most online vendors allow you to filter for the table and depth percentages in their advanced options!

Then, you will only have to look at the grading report of the diamond to make sure that the crown and pavilion angles also match my suggested proportions. After that you wil single out all the diamonds that you like most in terms of how they look and sparkle (every diamond has a unique sparkling style). Finally you should ask for an ASET analysis of the diamonds you are left with and pick the one with the best light performance. Online vendors such as James Allen, Brian Gavin and Whiteflash will even provide you with an ASET photography for up to three diamonds for free. In fact, I also use this option a lot when I am looking for the best diamonds for the visitors of my website.

This is the most effective way to go about picking a diamond with the best diamond proportions.

If you should have any more questions, just drop me a mail or leave me a comment!