The diamond cut is the most important of all the 4Cs and it is something different from the diamond shape. I am mentioning this directly at the beginning to avoid any confusion. I know from my own experience that many people use the words diamond shape and diamond cut interchangeably.

The diamond shape only refers to the outline of a diamond and the most common diamond shapes are listed below. The confusion between diamond shape and diamond cut arises because certain diamond shapes include the word “cut”. Thus the term “princess cut diamond” actually refers to the shape of a diamond but not to the diamond cut of the 4Cs.

The diamond cut rules everything

The diamond cut refers specifically only to round and princess cut diamonds and indicates how well-proportioned a diamond is. Therefore, you will not find any cut grades on diamonds other than round or princess cut diamonds. For fancy shaped diamonds you will have to look at the symmetry grade of a diamond and to look closely at the facet parameters to find out whether the light performance of such a diamond will be good.

The picture on the left depicts various parameters that are important in determining the cut of a diamond. The relationship between various angles of a diamond or the relationship between a diamond’s depth and diameter are decisive factors in determining a diamond’s cut.

The cut is the most important quality criterion in a diamond! It is the diamond cut that will decide in which way the light will traverse through the diamond and in which way it will be reflected back to the viewer’s eye. A good diamond cut will reflect the light in such a way that it will give a boost to all the other 3Cs.

The diamond cut and light performance

In fact, a well cut diamond reflects more light back to a human’s eye and thus makes the diamond appear larger than it actually is. Furthermore a well cut diamond reflects the light in such a way that the face up color appears brighter than it actually is. Moreover, a well cut diamond masks and camouflages inclusions better than a poorly cut diamond would. This is due to the fact that a well cut diamond has more scintillation and fire which can easily mask inclusions.

If you want to see the difference between a diamond with a perfect diamond cut and one with an inferior cut in action just compare these two diamonds: This diamond is exactly within the proportions for super ideal cut diamonds and exhibits a lot of sparkle, brilliance and scintillation. You can see that the sparkle and fire are perfectly balanced.

This diamond outside the super ideal proportions on the other hand looks much less lively and exhibits considerably less sparkle and scintillation.

One of the main factors in deciding how well the light will reflect back to the viewer is the pavilion angle. A too shallow or too steep pavilion angle will cause less light to be reflected back to the viewer:

Last but not least, a well cut diamond provides more fire, brilliance and scintillation. It sparkles much more and is much more impressing in terms of its bling factor! Thus, by choosing a diamond with an excellent cut grade you do not have to spend as much money on the other 3Cs and you will get a really impressing diamond.

Therefore, the diamond cut should be your absolute top priority in choosing a diamond. You can compromise on everything else, especially on the clarity and color of a diamond. But you should never compromise on the cut of a diamond!

Why do poorly cut diamonds exist?

But if all the above is true, why do poorly cut diamonds exist? You would think that people would not be interested in buying poorly cut diamonds when the return in brilliance, fire and scintillation is so much worse. After all, this is what we buy a diamond for!

In fact many diamonds are poorly cut on intention by diamond cutters. This may happen for various reasons. For one thing, many diamond cutters are intent on cutting a diamond from the rough in such a way that as much weight as possible will be preserved. Obviously, the thing that people will pay attention to most in a diamond is its carat weight. The first thing you will likely be asked when you have a diamond is usually its carat weight! Because people pay more attention to carat than to the cut, the prices increase more for more carat than for a better cut. Thus, diamond cutters have an incentive to not necessarily produce a diamond with the best possible cut grading.

If a diamond cutter produces a deeply cut diamond, more weight can be hidden in the diamond making the carat weight and thus the price go up. The cut grading will likely go down but in most cases it pays more for the cutter to go for more carat weight.

Furthermore, it also happens very often that diamond cutters want to eliminate a particularly striking inclusion. They do so by cutting the inclusion out, but of course they also want to preserve as much weight as possible. Once again, they have to do so by cutting the diamond in such a way that it is not perfectly symmetrical any more.

The different cut grades and which cut grade to choose

You should always look out for diamonds rated by either GIA or AGS. Please do not accept any other grading reports from other grading companies, especially not an inhouse grading report by the store you want to buy from.

GIA and AGS have different grading categories for cut. GIA categorizes cut grades into 5 categories whereas AGS divides diamonds into 6 different cut grades:

The grading systems of GIA and AGS are different and an AGS 0 does not equal a GIA Excellent. Still, GIA’s and AGS’s cut grades are a great indicator of the cut quality of a particular diamond.

My top recommendation is to go for the best cut! This means that you should choose a GIA “Excellent” diamond or an AGS “Ideal” diamond. In my opinion and in the opinion of many other experts the diamond cut is the wrong place to try to save money! You should save money in color and clarity. Many people pay for premium grades in color and clarity that cost considerably more but are not noticeable at all to the naked eye! A better cut grading on the other hand is noticeable to the naked eye!

Now, if you are really tight on your budget and you cannot find another way to save money in either carat, color or clarity it is still okay to choose a cut grade of “Very Good” for GIA rated diamonds or “Excellent” for AGS rated diamonds. Such a cut grade will still be okay but usually one can notice an increase in brilliance when going for Excellent/Ideal.

Visible differences between different cut grades

Even if it might still be considered okay to choose a GIA “Very Good” or AGS “Excellent” I would strongly advise anyone not to choose a cut grade below these two. The reason can easily be seen with your own eyes: Lower cut grades simply look terrible! But I don’t want you to believe me but I want you to see it with your very own eyes.

I will be showing you diamonds within the same cut category so that you can get an idea that there can be differences in diamonds within the cut grade. The following diamonds are all GIA graded diamonds:

Both diamonds are graded “Excellent” by GIA. The diamond on the left has nearly perfect proportions while the center of the right diamond is slightly off.

These two diamonds are both graded “Very Good” by GIA. The diamond on the left has exceptionally good proportions for its cut grade. The diamond on the right on the other hand might be considered below average for the “Very Good” cut grade. While the diamond on the left would still be an acceptable choice I would recommend no one to take the diamond on the right.

Whereas the diamond on the left is really poorly cut the diamond on the right is a complete mess. None of these two diamonds is a really acceptable choice.

Fair or even poor cut grades are so disastrous that in fact many shops do not even sell them! I hope you got the idea.

But looking at images will only help you so much. If you want to know what your diamond will look like you have to inspect it from all sides. Thankfully, James Allen has introduced its revolutionary 360° video display technology that lets you inspect all diamonds in high definition and magnified view. This prevents you from buying your diamond blind which is still what most people do nowadays online.

As you see, even if two diamonds share the same cut grade they do not necessarily have to appear the same. The reason is simply the fact that within the same grade both GIA and AGS allow for a certain level of variation as long as it is within the limits of the specific cut grade.

A diamond that is not 100 % symmetrical can still be very brilliant because most of a diamond’s brilliance is produced by the diamond cut and not by the symmetry. And a diamond’s symmetry is another quality criterion that I will be addressing later on. In many cases an excellent diamond cut and an excellent symmetry go hand in hand but they do not necessarily have to.

It is mainly the proportions between different parameters of a diamond that determine the diamond’s brilliance. If you know something about diamond proportions you will be able to pick the best diamond within the same cut grade!

How cut grades are determined

The diamond cut is determined by certain parameters that you will always find on the grading report! The cut is governed by the dimension and proportions of these parameters. These decisive parameters are the following:

• Table: Equals the “Table Percentage” which is measured as a % of the diameter.
• Star Facet: Equals the “Star Length” which is measured as a % of the distance between the girdle edge and the table.
• Crown: There is the so called “Crown Angle” which is measured as the angle the crown facet makes with the girdle. Furthermore there is the so called “Crown Height” which is measured as a % of the overall diameter.
• Upper Girdle: A measurement of the upper girdle is usually not to be found on lab reports.
• Girdle: On the Lab report the “Girdle Thickness” describes the thickness of the girdle and is measured as a % of the diameter.
• Lower Girdle: On the lab report the “Lower Girdle Length” is measured as a % of how far it extends toward the culet.
• Pavilion: On the lab report the “Pavilion Angle” is measured as the angle the pavilion makes with the girdle. Furthermore the so called “Pavilion Depth” is measured as a % of the diameter.
• Culet: Usually only the size of the Culet is described in the lab report

Now, all these parameters must lie within certain limits to get a specific cut grade. Today’s cut grades are very close to the Tolkowsky Ideal-cut. The Tolkowsky Ideal-cut was introduced by the mathematician and renowned gemmologist Marcel Tolkowsky in 1919. He published his findings for the perfect diamond proportions in his thesis entitled “Diamond Design, A Study of the Reflection and Refraction of Light in a Diamond”.

The Tolkowsky Ideal-cut consists of the following proportions:

Nowadays, GIA and AGS have slightly modified the proportions from the Tolkowsky Ideal Cut for diamonds with Excellent/Ideal light performance. You can find the values on the right. Please bear in mind though that these values should be used as a basic guideline. If you want to know the perfect parameters specifically for a round cut diamond please read my educational material about the round cut diamond.

Cut grades are determined by GIA and AGS according to the parameters of a diamond. Both GIA and AGS cut grading systems differ from each other in subtle ways. I have written an extensive article about the specific differences in the cut grading systems of GIA and AGS which anyone should read who is interested in it.

If you happen to have the parameters of a diamond at your disposal but not a reliable grading report by either AGS or GIA I would advise to type in these parameters into the free GIA facetware tool. This will quickly tell you the cut grade GIA would give to the diamond.

The above proportions will usually suffice to have a diamond with an Excellent/Ideal cut grade (AGS is stricter than GIA in cut grading). However, if you want to have a super ideal cut diamond ti should be within the following proportions:

Apart from the cut grades grading labs also give grades for symmetry and polish. The symmetry and polish grades are assigned independently from the cut grade to a diamond but they also have an impact on the cut grade. For instance the GIA guidelines declare that in in order to qualify for an Excellent cut grade, polish and symmetry of a diamond must be Excellent or Very Good. Please read on if you want to know more about symmetry and polish.

Read next: Diamond Symmetry