Diamond Cut Grade – Everything You Need To Know

When buying a Diamond, most people get a little obsessed with the color, clarity and sometimes the carat weight as these are the most commonly talked about (and often advertised) but arguably the most important of the 4 C’s is Cut.

And in this post, I’m going to cover all the aspects of cut so that you can not only learn why it is important but also some tips when it comes to buying a Diamond.

Key Takeaways

  • Only Round Brilliant Diamonds are currently cut graded
  • Diamond cut is the quality of the cut, not the shape of the stone
  • Diamond are graded on their cut quality (proportions), symmetry and polish
  • Each one is graded from Excellent to Poor
  • There are a range of ‘ideal’ parameters, especially for proportions

What is Cut in a Diamond?

Cut in Diamond refers to the quality of the cut (not the shape) and is based on the proportions, symmetry and final polish of the Diamond. The quality of the cut has a big impact on the look of a Diamond as a well-cut Diamond will have a lot more sparkle than a poorly cut Diamond, which can look dull and lifeless. 

The quality of the Cut also has an impact on a Diamond’s price as they better the quality of cut, the better the stone looks, which makes it more desirable.

One thing to note is that currently, only Round Brilliant Cut Diamonds are cut graded by the GIA. 

What Are The Diamond Cut Grades?

GIA Cut Scale

The most commonly used grading system for Diamond cut is the the one that was released by the GIA in 2005, following 15 years of research and includes 5 grades, which are:

  • Excellent 
  • Very Good
  • Good
  • Fair
  • Poor

But how does the GIA assign a Diamond one of these cut grades? 

When it comes to giving a Round Brilliant Cut Diamond a Cut grade, the GIA does this based on the following seven factors:

  1. Brightness – this is how much white light is reflected both internally and externally from the Diamond
  2. Fire – this is how well the Diamond disperses light into the colors of the rainbow
  3. Scintillation – this is a combination of how much sparkle the Diamond has and the pattern of light and dark areas within the stone, which are caused by reflections
  4. Weight Ratio – this is the comparison of the Diamonds diameter to its weight
  5. Durability – this is the potential risk of damage to the Diamond as mostly focused around the girdle (thin girdles are more likely to get chipped)
  6. Polish – this is the quality of the final polish that has been applied to the facets
  7. Symmetry – this is a combination of proportional symmetry and facet symmetry

And each of these 7 factors are given individual grades of Excellent to Poor but the overall cut grade is based on the first 5 and the grade it is given is based on the lowest grade given to any of these five, for example:

  • A Diamond could have excellent brightness, scintillation, weight ratio and durability but it only gets a good grade for fire, then the overall cut grade for that Diamond will be good

Polish and Symmetry grades don’t impact on the overall cut grade and are noted separately on Diamond reports. 

The challenge with giving a Diamond a Cut Grade is that there are many variables involved (much more than any of the other 4 C’s) and in order to do this, a combination of 3D scanning and visual inspection by a trained Diamond grader is required. 

Factors That Affect Diamond Cut

I know I just covered the 7 factors that GIA use when assigning a cut grade to a Diamond but the 5 that are used the asses overall cut quality are a little bit vague in my opinion and in reality, there are individual factors that are looked at on a Diamond that are then combined to come up with the broader ones that GIA use. 

And in this section, I’m going to look at these in more detail as understanding the different things that are listed on Diamond reports can help make the buying process a lot easier as you aren’t being bombarded with jargon and numbers, so let’s look at these factors in a bit more detail…

1. Proportions

The most important factor when it comes to assessing the cut quality of a Diamond are the proportions (called Cut Quality on GIA Reports) as these directly impact on how much fire and sparkle a Diamond has.

Anatomy of a Diamond Cut

The overall Cut Quality is made up of 10 individual factors but the challenge with grading a Diamonds cut quality is that there isn’t a single set of ‘ideal proportions’, instead there is a range for each of these 10 factors, which is why you can have two Excellent Cut Diamonds that look different. 

But what are these 10 factors and how do they impact the look of a Diamond? 

Average Girdle Diameter

Before I get into these 10 factors, there is one very important measurement that needs to be taken and this is the Average Girdle Diameter as 3 other proportions are in relation to this and the reason why it is called to average girdle diameter is that very few Round Brilliant Cut Diamonds are truly round, they usually have slight variations in length and width. 

To calculate this, the Diamond is measured across multiple directions, this is usually north to south, east to west and the two diagonals and then an average is taken from these four measurements.

1. Table Size (Ideal 52% – 62%)

Diamond cut proportions - table size

One of the most important proportions is the table size and this is the size of the table in relation to the average diameter of the Diamond and the size of the table plays an important role in balancing:

  • The amount of brilliance a Diamond has, which is the amount of light that is returned to the eye (depending on the pavilion)
  • The amount of fire a Diamond has, which is how much light is dispersed by the crown facets (the rainbow colors that can be seen)

If the table is too small, the Diamond can have a lot of fire but doesn’t look very bright as the amount of light being returned from the stone is limited. 

If the table is too big, the Diamond can return a lot of light but can look a bit lifeless as the smaller crown facets struggle to produce much fire. 

2. Crown Angle (Ideal 31.5 – 36.4)

Diamond cut proportions - Crown Angle

The crown facets are responsible for producing the fire seen in Diamonds (the rainbow colors) but they have to have the right angle in order to maximize this and in most cases, the crown angle is closely related to the table size as:

  • A Diamond with a large table can result in the crown facets being at a steeper angle, which reduces the fire in the Diamond
  • A Diamond with a small table can result in the crown facets being at a shallower angle, which increases the fire in the Diamond

So the crown angle combined with the table width has a big impact on the face-up appearance of the Diamond, with stones that fall in the ideal or excellent range exhibiting the best balance of fire and brilliance, meaning that the Diamond is more likely to be lively and sparkly. 

Diamond cut proportions - ideal table size
Ideal table size: Good mix of fire and brilliance
Diamond cut proportions - large table
Large table: More brilliance but less fire
Diamond cut proportions - small table
Small table: More fire but less brilliance

3. Crown Height (Ideal 12.5% – 17%)

Diamond cut proportions - Crown Height

The crown height is measured from the table to where the crown meets the girdle and is represented as a percentage of the average girdle diameter. Crown height is very closely related to the table size and the crown angle as it needs both of these to fall within the ideal range in order for the crown height to also be ideal. 

If a Diamond has a crown height that is outside of the ideal range, then it will more than likely have:

  • A table that is too large or small
  • A crown angle that is deeper or shallower than ideal

Which as I said above, reduces the amount of fire and brilliance that is seen in the Diamond. 

4. Pavilion Angle (Ideal 40.6 – 41.8)

Diamond cut proportions - Pavilion Angle

The pavilion angle is one of the most important proportions of a Diamond as this determines how much light is reflected back out of the stone and in an ideal world, cutters would aim for what is known as Total Internal Reflection (TIR), which means that all light entering the stone would be returned. 

But this isn’t always the case as some Diamonds are cut with commercial considerations in mind, which can result in Diamonds having either too shallow or steep pavilion angle and this results in light leaking out of the stone, which can not only reduce the amount of fire and brilliance seen in the Diamond but may also be noticeable when looking at the stone as:

  • A Diamond with a shallow pavilion angle can result in the girdle being reflected in the table of the stone, causing an effect known as fish-eye
  • A Diamond with a steep pavilion angle can make the table appear dark, which is sometimes called a nailhead

Neither of these are ideal and with Diamonds with steep pavilion angles, you may be paying for additional carat weight that actually makes your Diamond look worse. 

Diamond cut proportions - ideal pavilion angle
Ideal Pavilion Angle: Light reflected out of the top of the stone
Diamond cut proportions - shallow pavilion angle
Shallow Pavilion Angle: Light leaks out of the bottom of the stone
Diamond cut proportions - steep pavilion angle
Steep Pavilion Angle: Light leaks out of the pavilion of the stone

5. Pavilion Depth

Diamond cut proportions - Pavilion depth

The pavilion depth is directly related to the pavilion angle as the steeper the pavilion angle, the greater the pavilion depth and is the measurement from where the pavilion meets the girdle to the culet, this is represented as a percentage of the total average diameter of the Diamond on a report. 

6. Girdle Thickness (Ideal Thin – Slightly Thick)

Diamond cut proportions - Girdle thickness

The girdle thickness is another important proportion but not so much in terms of how the stone looks as some of the ones mentioned above but more in terms of durability and how heavy the stone is in relation to its diameter. 

Girdle thickness can range from extremely thin (sometimes called a knife edge) to extremely thick and the extreme ends of the scale are to be avoided because:

  • Extremely thin girdles can cause durability issue as the girdle is that thin that it can easily be chipped or damaged when the stone is being set or worn
  • Extremely thick girdles can add a lot of weight to the stone, which you pay for but don’t get a stone that has a smaller diameter than the same sized stone with a very good or ideal-sized girdle 

Very thick girdles can also cause light to leak out of the stone, which does have a negative impact on the look of the stone but not to the extent of table size, crown or pavilion angles. 

7. Total Depth (Ideal 57.5% – 63%)

Diamond cut proportions - Total depth

The total depth is the depth of the Diamond in relation to the average girdle diameter and if a Diamond has a total depth within this ideal range, it increases the chances of the:

  • Crown height
  • Pavilion depth
  • Girdle thickness

All fall within the ideal ranges for their respective proportions but this isn’t always a guarantee. 

8. Culet Size (Ideal: None – Small)

Diamond cut proportions - Culet size

Not all Round Brilliant Cut Diamonds have a culet, which is a facet at the very bottom of the stone and most Round Brilliant Diamonds either don’t have one or it is so small that it is very difficult to see with the naked eye and while this is more desirable, it does increase the chances of the point getting damaged, especially before the Diamond is set! 

But some Diamonds will have a noticeable culet and this isn’t ideal as it can create a ‘window’ in the stone, which means that the light is passing straight through the stone rather than being reflected back out of the table.

9. Star Facet Size (Ideal: 45% – 65%)

Diamond cut proportions - Star facet size

A relatively recent addition to Diamond cut grading is the size of the star facets as being too long or short can impact the way that light interacts with the Diamond. 

The size of the star facets is measured as a percentage of the distance between the girdle and the edge of the table. 

10. Pavilion Facet Size (Ideal: 70% – 85%)

Diamond cut proportions - Pavilion facet size

As with the star facets, the size of the pavilion facets or lower half facets as they are also called is now a part of the cut grading process as if these facets are too long or short, then impact on how the light interacts with them, which can have an impact on how the stone looks. 

The length of the pavilion facets is measured as a percentage of the distance between the girdle and the culet of the stone. 

2. Symmetry 

The second main factor of grading the cut of a Diamond is how symmetrical the stone is and the best way to think about this is to picture a line going down the center of the stone and does the left side look the same as the right?

When it comes to symmetry, it is broken down into two main categories:

1. Proportional Symmetry 

Proportional symmetry is based on what we just talked about but there are times where aspects of these proportions aren’t symmetrical and this can have an impact on how the Diamond looks such as affecting the fore and or brilliance, these include:

  • A titled table
  • The table being off-center
  • The culet being off-center
  • An uneven or wavy girdle

All of these can impact on how light interacts with the Diamond and depending on the severity of the symmetrical errors, can have a negative impact on the Diamond’s appearance. 

2. Facet Symmetry

The second type of symmetry is down to how the Diamond has been faceted and includes:

  • Additional facets 
  • Facet edges not meeting up properly
  • Uneven facet lengths 

As with the proportional symmetry, this can have an impact on how the Diamond interacts with light but in some cases may not be as big of an impact. 

3. Polish 

The third factor when it comes to cut quality is how good the final polish is on the Diamond and while many Diamonds are polished to a perfect or near-perfect finish, there are some that can have:

  • Polishing lines on the facet faces
  • Burn marks on the surface of the Diamond 
  • Areas that haven’t been polished

These polishing imperfections may be noticeable but in most cases need to be viewed under magnification to see. 

What You Need To Know About Diamond Cut

It is very easy (especially as a gemologist) to talk about all the technical aspects of Diamond Cut and I hope that I’ve made this post pretty easy to follow but when it comes to buying a Diamond, there are some things that you need to know that can make the whole buying process a whole lot easier:

1. If Possible, Go For Excellent or Very Good Cut Grades

This is because these Diamonds have been graded as performing well in all the factors that are used to assess a Diamonds cut quality, meaning that the Diamond should be bright and sparkly due to having good fire and brilliance. 

You should also go for Excellent or Very Good for the symmetry and polish. 

2. Two Diamonds With The Same Cut Grade Can Look Different

As you will have seen earlier in this post, with each of the factors that are used to grade the cut quality of a Diamond, there are a range of parameters rather than a single set of figures for the ‘perfect’ cut Diamond and this means that you can have two Excellent Cut Diamonds that look different when viewed side by side. 

This is part of the beauty of Diamonds as each one is unique and has its own characteristics.

Conclusion

The cut quality of a Diamond is so important and could be argued that it is the most important of the 4 C’s as it has such a big impact on how good the Diamond looks and after reading this, you will have learned that:

  • Currently, only Round Brilliant Diamonds are cut-graded
  • The proportions of a Diamond impact on how the stone interacts with light
  • The symmetry and final polish of the Diamond are also quality factors

And if you are buying a Diamond, then cut quality should be considered just as you would color and clarity as combined, they have a big impact on how your Diamond looks.

I'm Paul Haywood FGA DGA, the owner and founder of Diamonds and Dials, I'm a fully qualified Gemmologist and Diamond Grader from the Gemmological Association of Great Britain.

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