Diamond Cut Quality and Precision

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Visual Performance is Controlled by Cut Precision.
Cut Precision = Polish, Symmetry, Proportions, and Optical Precision.
"Shape" is Not The Same As "CUT"

Diamond Rough

When a diamond comes out of the ground it looks like a shiny pebble or piece of quartz. As a matter of fact, it looks so unimpressive that you probably wouldn't think it was a diamond.

The first diamond discovered in South Africa (not the world) was found in 1856. A young shepherd named Klonkie hurled it at one of his friends named Erasmus Jacobs.

According to legend, the young man pocketed the rock and gave it to his sister to play with. Quite some time later, a visitor to the Jacobs home was admiring the rock that was sitting on the mantle of the fireplace.

He thought that the specimen might be a diamond and got permission to take it for further study. Eventually, it was confirmed that the rock was a diamond weighing 21.25 carats. The diamond was cut into what is now the 10.73 carat, oval brilliant Eureka Diamond.

Precision planning and cutting is necessary to unlock the hidden beauty of a diamond. The brilliance, fire, and scintillation that make diamonds so desirable is the result of how skillfully a diamond is cut.

With this in mind, it's time to discover why diamond cut quality is the most important and least understood of the 4C's of Diamond Grading.

Diamond Shape & Diamond Cut = Different Things:

Blue Nile Studio Engagement Rings.

Blue Nile Studio Engagement Rings.

Lay people often confuse the SHAPE of a diamond with its CUT Quality. This is most likely due to the industry's synonymous use of the terms "cut" and "shape" to describe the diamond shape (outline).

For example, we use terms like marquise cut, round brilliant cut, princess cut, Asscher cut, or pear shape in reference to the shape.

In reality when we refer to the "cut" or "the make" of a diamond, we are really referring to a compilation of three factors which combine to determine the Overall Cut Grade of the diamond:

  1. 1
    Proportions, the angles of the facet plane described in degrees and percentages.
  2. 2
    Polish, the finish quality of the diamond.
  3. 3
    Symmetry, the consistency of facet shape and alignment.
  4. 4
    Optical Precision, the consistency of facet shape, size, and alignment from the perspective of 360 degrees.

Diamond Cut Quality (The Most Important 4C):

If you remember only one thing from this site, remember this: 98% of a diamond's visual performance (brilliance, dispersion, scintillation) is dictated by the cut quality. Diamond clarity, color, and carat weight contribute to the price but have no affect on sparkle factor or light performance.

This is why I spend so much time focusing on the degree of optical precision, proportions, symmetry, and polish grades. The remaining 2% of the equation is mostly in your head in terms of what you prefer from the perspective of keeping things mind-clean.

F-color vs K-color Diamond Comparison.

F-color vs K-color Brian Gavin Signature diamonds.

People frequently make the mistake of assuming that a D-E-F color diamond will be more brilliant because the color spectrum is brighter. In reality, the terms brilliant and brighter represent different things as it applies to diamonds.

Brilliance is the white sparkle that is created by light reflecting off the facets of the diamond. While brightness more accurately describes a spectrum or range of hue and saturation.

As such, it's true that a D-E-F color diamond may face-up whiter and brighter than one that is G-H-I-J color. However, the I-color diamond will exhibit more sparkle and better light performance if it is cut better.

As a matter of fact, none of the other factors such as Carat Weight, Color, or Clarity, have a more dramatic impact on sparkle factor than the cut quality.

Aspirations Versus Ideal Cut Proportions:

There are some days when I imagine that Ali Baba & The 40 Thieves have stolen a box of micrometers. Obviously, it is their evil intent to mislead you into believing that every round brilliant cut diamond is ideal cut. Can you say camel dung?

Now we're not saying that everybody in the jewelry industry is a thief, or a member of Ali Baba's gang. However there doss seem to be an awful lot of people in the diamond industry who rely upon a much looser definition of the term "Ideal Cut" than we do. As a matter of fact, the range of proportions for the AGS Ideal and GIA Excellent cut grades is much broader than we find tolerable.

With that in mind, a lot of the diamonds described as AGS Ideal or GIA Excellent are laughable at best by our standards. Let's face facts, even the Tooth Fairy knows that a round brilliant cut diamond with a Table Diameter of 60% and a Total Depth of 60% is not an ideal cut diamond. Especially when the polish and symmetry grades of the diamond are only Good/Good.

However, you probably haven't been buying diamonds as long as the Tooth Fairy has been tinkering about. Therefore, we're going to debunk the fairytale for you. In your quest for perfection, you're likely to run across terms like:

  • Signature Ideal.
  • European Ideal.
  • American Ideal.
  • 60/60 Ideal Cut.
  • Tolkowsky Ideal Cut.

The Ultimate Ideal Standard for Diamonds:

In the first place, none of the terms above mean anything without a defined set of parameters by which to compare them. We base our use of the term "Ideal Cut" upon the grading system developed and used by the American Gem Society Laboratory (AGSL).

That is because we find it to be a more reliable standard.This is especially true when the diamonds are graded on the proprietary light performance grading platform that incorporates the Angular Spectrum Evaluation Tool (ASET).

As a matter of fact, Brian Gavin adheres to the AGS Light Performance grading standards. With this in mind, it should be no surprise that they are both former suppliers of ours. By the way, Brian Gavin is the only diamond cutter in the world who holds a patent for light performance in the modern round brilliant cut diamond.

Defining Ideal Cut Parameters:

Regardless of the term used to describe any diamond you might be offered, it is possible for you to compare the actual proportions of the diamond to the AGS defined standard for "ideal proportions" by simply obtaining a computerized proportions analysis of the diamond in Sarin, OGI or Helium format.

Subtle and minute dimensional differences of a degree or so can determine whether a diamond will be full of life or dull and listless. With that in mind, we'd like to introduce you to the concept of "Ideal Cut" diamonds. That is, diamonds cut within a specific range of "ideal proportions" that consistently deliver maximum brilliance, dispersion and scintillation. 

Just as you can reap the benefits of a great haircut, or look like a dork for six weeks, diamonds look their best when they have been cut to precise proportions. Diamonds cut to these precise proportions are called "Ideal Cut" and are certainly worth looking for, especially since you don't have to look any further than the vendors represented on this web site.

Diamond Proportions Grading By The Numbers:

The original Cut scale developed by the AGS consists of eleven categories that describe the proportions of a round brilliant cut diamond. The highest rating is AGS Ideal-0 and the lowest is AGS-10 Poor.

The overall cut grade of the diamond is determined by taking all of the factors:

  1. 1
    Light Performance.
  2. 2
  3. 3
  4. 4

A diamond that receives the overall cut rating of AGS Ideal-0 if the grade for all four categories meets the criteria for ideal cut. As a matter of fact, Brian Gavin is the only diamond cutter who holds a patent on maximizing light performance in the modern round brilliant cut diamond.

AGS Ideal-0 vs AGS-000 and Triple Ideal Cut:

In the early days of the AGS Laboratory, a diamond graded as having AGS Ideal-0 Polish, Symmetry, and Proportions was commonly referred to as an "AGS-000" or "Triple Zero Ideal Cut" diamond. This description is not accurate because the AGS Laboratory also uses a numerical system to describe color and clarity grades.

Thus, a true "triple zero diamond" on the AGS grading scale would be Flawless in clarity, D-color, and have an overall cut grade of AGS Ideal-0. With this in mind, if the salesperson you are working with describes anything else as "AGS-000" or "triple zero" then the odds are that they don't know what they're talking about. Either that or they happen to be several years behind the times and have yet to change their lingo.

Ant 20x Magnification

Regardless of what you call it, a Super Ideal Cut Diamond will make the hairs on a gnats posterior stand up and quiver. By the way, that’s a politically correct colloquialism for those of you who can’t handle the word “ass”.

In case you're wondering, we said that to a client once and he pressed us for further explanation. Since we couldn't catch a gnat without squishing it, we opted to photograph this California black ant. This is what it looks like as seen through our Gem Scope at 20x magnification.

No ants were harmed or killed in the production of this photograph.

Defining Ideal Parameters for Non-AGS-graded Diamonds:

To properly determine the proportions rating of a diamond, it is mandatory that you know the following details:

  • Table Diameter %.
  • Crown Angle in Degrees.
  • Pavilion Angle in Degrees.
  • Girdle Thickness %.
  • Culet Size.

Note that Pavilion Angle in Degrees is NOT the same as the Pavilion Depth that is expressed as a percent. In addition, the AGS and GIA gemological laboratories determine the measurements stated on the lab reports differently. You can read more about that in this article on the AGS versus the GIA Laboratory.

The Original AGS Proportions Scale:

The original Cut scale developed by the AGS in the mid-1990's consists of eleven categories. These categories are used to describe the proportions of a round brilliant cut diamond.

The AGS Laboratory introduced their proprietary Light Performance grading platform in June of 2005. As it so happens, that was the same time the GIA Laboratory announced that they would be adding an overall cut grade to the diamond grading reports for round brilliant cut diamonds.

Only the AGS Laboratory grades diamonds for light performance and the GIA still does not provide crown or pavilion measurements for fancy shape diamonds. From our perspective, the GIA Gem Trade Laboratory is seriously behind the times. However, we still respect their position of authority and all the contributions they have made to our industry.

For your convenience the broad spectrum parameters for the original AGS Ideal Cut proportions rating for round brilliant cut diamonds is as follows:

  • Table Diameter: 52.4 - 57.5%
  • Crown Angle: 33.7 - 35.8° degrees.
  • Pavilion Angle: 40.15 - 41.20 degrees.
  • Girdle Thickness: Thin, Medium, Slightly Thick.
  • Culet Size: None, Pointed, Very Small, Small, or Medium.

The Proportions Parameters Above Are Too Broad!

We want to state clearly that the range of proportions stated above are too broad by our standards. Those measurements represent the original specifications for the proportions grade that was available from the AGS Laboratory from 1996 through June of 2005.

The new proportions grade that is used in conjunction with the Light Performance analysis is much more in-depth. As a matter of fact, it is too complicated to try and explain in a few paragraphs. In the first place, a different range of proportions is used depending on the table diameter measurement of the diamond. In the second place, this tutorial on the Angular Spectrum Evaluation Tool (ASET) is a book in itself.

Note that the Total Depth % of the diamond is not taken into account. This is because it is the Crown & Pavilion angles that control most of the brilliance. Thus, the Total Depth is not as important a factor. At the same time, we prefer that the total depth of a round brilliant cut diamond be somewhere between 59.0% and 61.8%.

What About a 62% Total Depth?

Of course, you can stretch things up to about 62% without affecting things too much. However, we really wouldn't go up to 62.3% and 62.5% is really stretching it from our perspective.

With this in mind, it is not uncommon to find AGS Ideal Cut diamonds with total depths as deep as 63.5%. Of course, we recommend that you avoid them like the plague because they are simply too deep! There is a direct relationship between the outside diameter and the total depth of a diamond.

The deeper the total depth, the smaller the outside diameter of the diamond will be for a specific range of carat weight. For instance, a properly cut 1.00 carat round brilliant ideal cut diamond should have an outside diameter in the range of 6.50 mm with a slight variance such as 6.49 - 6.53 mm or thereabouts.

If the diamond had a deep total depth, such as 63.5% then the outside diameter might be more like 6.3 mm in which case you might be paying a one carat price for the look of a diamond which "faces up" like a 0.90 carat diamond. While the diamond might be vibrant and full of life, the fact is that there is a price increase that occurs between the 0.99 - 1.00 carat marks.

With that in mind, it's just bad business to pay a one carat price for the look of a ninety pointer. The concept holds true for diamonds in other weight categories, diamond prices increase based upon Price Per Carat (PPC) at various junctures of weight.

AGS and GIA Proportions Models Are Drawn To Scale:

You can find the proportions diagram on the lab report on the diamond grading report. The proprietary report issued by the AGS for Black by Brian Gavin Diamonds features the proportions diagram on the left side.

The proportions diagram is shown to scale based on the actual measurements of the diamond. Note that the crown angle, pavilion angle, star facet, and lower girdle facet length measurements represent the average of eight measurements taken per section.

The AGS Laboratory reports the average measurement per section based on adding up the eight measurements and then dividing that sum by eight. Whereas the GIA further rounds those measurements off to the nearest half a percent, half a degree, and up to the nearest 5% for star length and lower girdle facet length.

Obviously, there are potential issues with how diamonds are measured. Just think about it for a moment.

If the average crown angle measurement indicated on the lab report is 34.5 degrees and the lowest crown angle measurement is 34.3 degrees and the highest is 34.8 degrees, that is great! However, if the low measurement is 33.5 degrees and the high measurement is 35.5 degrees, that is less than desirable.

Okay, actually, that diamond is just going to suck, but we see it all the time and the average measurement indicated on the graph provided on the lab report would still be 34.5 degrees. This is why it's so important to analyze the reflector scope images and why we're so excited about the implementation of Advanced ASET.

Tolkowsky Cut and Tolkowsky Range:

The next trick that Ali Baba and his den of thieves likes to rely upon is the infamous tale of Marcel Tolkowsky. A physicist and member of a Belgian diamond cutting family, who in 1919 published Diamond Design.

It was the first major analysis of diamond proportions. His work was based on [what were then] modern theories of light behavior and his opinion of what proportions resulted in the best possible balance of brilliance and dispersion of light.

Ali Baba likes to say things like "This diamond has Tolkowsky proportions, but I didn't want to increase the price of the diamond by having it certified". This is usually bunk, because if all of the proportions for the diamond met Tolkowsky's parameters, they would have sent it in to be lab graded.

After all, then they would be able to get a better price for the diamond. By the way, none of the independent gemological laboratories actually "certify" anything. The labs only report the characteristics of the diamond based on their opinion at the time the diamond was graded. Thus, anytime uses of the term "certified" they are trying to mislead you into believing that the diamond is something it is not.

Tolkowsky Proportions (and what's wrong with them):

Tolkowsky's calculations for the ideal angles and proportions for a round brilliant cut diamond are as follows:

  • Table Width 53%
  • Total Depth 59.3%
  • Crown Angle 16.2% or 34° degrees.
  • Pavilion Depth 43.1% or 40.9°
  • Extremely thin knife edge girdle.

Notice that Tolkowsky specified that the girdle edge of the diamond be "Knife Edge" or virtually non-existent. That's great in theory, however many a girdle edge has been chipped because it was extremely thin and could not withstand the pressure of being set.

Interestingly enough, a knife edge girdle would result in a rating of AGS-10 Poor on the AGS Proportions Scale. Perhaps this is why we have yet to see a diamond that is cut to Marcel Tolkowsky's true standards. Instead what we see are a lot of diamonds cut within "tolerance of Tolkowsky's standards" or "Tolkowsky Range".

Using Sarin DiaMension computerized proportions analysis, we have evaluated a variety of "Tolkowsky Cut" diamonds. Of course, these diamonds feature more realistic girdle edge measurements. The results show that the proportions of a Tolkowsky cut diamond can range from AGS Ideal 0 to AGS-2 Very Good.

Therefore, we recommend you only consider a "Tolkowsky Cut" diamond when you have all of the information required to determine the actual proportions of the diamond. We feel that the parameters of the current AGS Ideal Cut rating system are far more accurate than what Tolkowsky created on his chalkboard seventy years ago.

AGS Proportions Scale - Historical Parameters:

As stated previously, the AGS Laboratory implemented the Light Performance grading platform in June 2005. At that time, they also updated their proportions criteria. The old scale appears below as a historical reference because we still find it insightful.

However, it is important to note that the updated proportions scale provides a different range of crown and pavilion angle measurements depending on the table diameter. Just the same, we thought that you might find the parameters for the original AGS-1 Excellent through AGS-3 Good range of interest.

The average light return for a diamond with AGS Ideal Polish, Symmetry and Proportions is around 96%. The average light return for a diamond with AGS-4 Good Polish, Symmetry and Proportions is somewhere around 70% which might help you to understand why we prefer ideal cut diamonds.

AGS-1 Excellent Proportions:

  • Table Width: 51.4 - 52.3% or 57.6 - 59.5%
  • Crown Angle: 32.7° - 33.6° or 35.9° - 36.3°
  • Pavilion Depth: 43.9 - 44.3%
  • Girdle Thickness: Very Thin.
  • Culet Size: Slightly Large

AGS-2 Very Good Proportions:

  • Table Width: 59.6 - 61.5%
  • Crown Angle: 32.2° - 32.6° or 36.4° - 36.8°
  • Pavilion Depth: 41.7 - 42.1% or 44.4 - 44.8%
  • Girdle Thickness: Very Thin to Slightly Thick.
  • Culet Size: Slightly Large or Smaller.

AGS-3 Good Proportions:

  • Table Width: 50.4 - 51.3% or 61.6 - 63.5%
  • Crown Angle: 31.7° - 32.1° or 36.9° - 37.3°
  • Pavilion Depth: 41.7 - 42.1% or 44.4 - 44.8%
  • Girdle Thickness: Thick.
  • Culet Size: Large.

Wrapping It All Up:

Unless you're comparing diamonds graded by a laboratory, it might not be easy for you to determine the actual proportions or cut rating of a diamond. Most diamond dealers and jewelry store owners actually don't calculate the proportions of the diamonds that they buy and sell.They select them based on how brilliant they look and price them accordingly.

If a diamond looks dull and lifeless, an experienced diamond grader knows that it's poorly cut. The diamond will sell easily to an inexperienced buyer who only considers size, clarity, color and price when shopping for a diamond. A lot of companies on the net rely heavily on this fact. Hence the abundance of on-line diamond search engines and long inventory lists of diamonds for sale with limited information.

Have you ever noticed how the crown angles and pavilion depth measurements are seldom listed for you to compare? Not to worry, this information is readily available on AGS and GIA graded diamonds and many vendors also supply the images necessary to judge light performance. Of course, we are happy to assist you narrow down the field of options by lending you a professional eye and years of expertise.

Nice Ice Blueprint™

The Nice Ice® Blueprint™.

Super Charge Your Diamond Buying Skills:

Follow the Steps in The Nice Ice® Diamond Buying Blueprint™ and Unlock the Secret for Maximizing Sparkle Factor and Light Performance. These are the Tips & Tricks from the back office of Nice Ice Diamonds.