Diamond Cut Quality Essential Performance

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Diamond Cut Quality is the most important characteristic of the 4Cs. The Gemological Institute of America (GIA) evaluates Polish, Proportions, and Symmetry as the basis for the overall grade.

The American Gem Society Laboratory (AGSL) also measures Light Performance. Their proprietary Angular Spectrum Evaluation Tool (ASET) was developed in cooperation with the University of Arizona.

Unfortunately, both methods of evaluating diamond cut quality fall short. That's because they do not account for optical precision. That is the consistency of facet shape, size, and alignment. Consequently, we evaluate optical precision from the perspective of 360-degrees.

Blue Nile Diamond Cut Quality Shines Through.

Blue Nile Diamond Cut Quality Shines Through.

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

Optical Precision & Diamond Cut Quality Are Essential To Performance:

Custom Pave Diamond Engagement Ring by Brian Gavin.

Custom Pavé Setting by Brian Gavin.

In the first place, the pavilion angle dictates the volume of light return. At the same time, the crown angle determines the balance of brilliance and dispersion.

If the crown and pavilion angle offset is within the right range, then the stage is set for a great show. However, the degree of optical precision or lack thereof will determine whether the show is a success.

In fact, the consistency of facet structure will decide how much light reflects upward toward the observer or leaks out of the stone. Under those circumstances, it's hard to understand why the labs ignore such a critical performance factor.

Diamond Cut Quality Unlocks the Beauty:

Diamond Rough

Blocking the Rough Diamond Crystal.

A diamond looks like a shiny pebble or quartz when it comes out of the ground. In fact, it looks so unimpressive that you probably wouldn't think it was a diamond.

In 1856, a young shepherd named Klonkie found the first diamond in South Africa (not the world). Supposedly, he hurled it at a friend named Erasmus Jacobs.

The young man pocketed the rock and gave it to his sister to play with. A visitor to the Jacobs home was admiring the rock sitting on the fireplace's mantle sometime later.

Sparkle Takes Skill and Planning:

Oval-shaped diamond engagement ring from James Allen.

Oval-shaped Diamond e-Ring by James Allen.

The guest was actually a geologist who thought that the specimen might be a diamond. In that event, he got permission to take it for further study. Eventually, they confirmed that the rock was a rough diamond weighing 21.25 carats.

Ultimately, the diamond was cut into what is now the 10.73 carat, oval brilliant Eureka Diamond. Precision planning and cutting are necessary to unlock the hidden beauty of a diamond.

It takes skillful planning and craftsmanship to unlock the brilliance, fire, and scintillation that make diamonds so desirable. That is why diamond cut quality is the most important and least understood of the Diamond 4Cs.

Diamond Shape & Diamond Cut Quality Are Different:

Lay people often confuse the SHAPE with Diamond CUT Quality. This is most likely because the industry uses the terms "cut" and "shape" synonymously.

For example, here is a list of diamond shapes:

Diamond Shape Is the Outline:

Blue Nile Studio Engagement Rings.

Blue Nile Studio Engagement Rings.

Technically speaking, the diamond shape is the outline and nothing more. However, the Diamond 4Cs imply that Cut is synonymous with Shape. Be that as it may, diamond cut quality encompasses a broad spectrum of characteristics, including:

Each of those characteristics is a subject worthy of its own tutorial. The outline or diamond shape is also a factor of symmetry.

Defining Diamond Make:

At the industry level, we refer to "diamond cut quality" as the make. Several factors contribute to the make of a diamond, including:

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

The cumulative effect of these characteristics defines the diamond make. For example, a diamond dealer might say, "this diamond has very fine make." However, the statement itself is of no value without factual data to support that claim.

The Most Important 4C Is Diamond Cut Quality:

Brian Gavin Select Cushion Diamond Cut Quality.

Brian Gavin Select Cushion Diamond Cut Quality.

Notice that I repeatedly emphasize that diamond cut quality is the most important of the 4Cs. That's because 98% of visual performance results from the overall cut quality.

Remember that nothing is more important than diamond cut quality. Consequently, the two most important factors are proportions and optical precision. A diamond is not going to reflect light well if the pavilion angle is off.

Nor will it deliver a balance of brilliance and dispersion if the crown angle is too shallow or steep. Any variance in the consistency of facet structure will also create light leakage. These Brian Gavin Select Cushion cuts look spectacular because of the diamond cut quality.

Engaging Sparkle From Across the Room:

It's natural for people to focus on the common characteristics of quality. After all, that is what the industry talks about most. However, these factors do not affect the sparkle factor or performance.

Consequently, the overall cut quality can affect diamond prices up to sixty percent. Of course, the remainder of the 4Cs also factors into the price.

However, a slight difference in carat weight, color, or clarity will not be apparent from across the dinner table. In contrast, the sparkle factor and light performance are visible from across the room.

This is why I spend so much time focusing on the diamond cut quality. In other words, the degree of optical precision, proportions, symmetry, and polish grades.

The remaining 2% of the equation is mostly in your head. In other words, the combination of carat weight, color, and clarity is a mind-clean thing.

Are Colorless Diamonds More Brilliant?

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. Perhaps that is because the color spectrum is brighter. However, the terms brilliant and brighter represent different concepts with diamonds.

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

Indeed, a D-E-F color diamond looks whiter and brighter than G-H-I-J color diamonds. However, an I-color diamond may exhibit more sparkle and better performance if it is cut better. By the way, the example of an F-color diamond is my daughter-in-law's Black by Brian Gavin Diamond.

Once again, diamond cut quality has more impact on the sparkle factor than any other factor of the 4Cs. Under those circumstances, I recommend focusing on proportions and optical precision above all else.

Aspirations Versus Ideal Cut Proportions:

Blue Nile Bezel Solitaire Diamond Cut Quality.

Blue Nile Bezel Solitaire Diamond Cut Quality.

There are times when I imagine that Ali Baba & The 40 Thieves have stolen a box of micrometers. Their evil intent is to mislead you into believing that every round brilliant cut diamond is ideal.

Can you say camel dung? Now we're not saying that everybody in the jewelry industry is a thief. Nor is every diamond dealer a member of Ali Baba's gang. However, many tradespeople share a broader definition of "Ideal Cut" than we recommend.

As a matter of fact, the range of proportions for the AGS Ideal and GIA Excellent cut grades is significantly broader than we can tolerate. In that case, many AGS Ideal and GIA Excellent cut diamonds are laughable by our standards.

Let's Face Facts & Debunk the Myths.

Even the Tooth Fairy knows that a "60-60 Ideal cut diamond" cannot compete with AGS Ideal-0. The idea that a diamond will perform well simply because it has a 60/60 total depth and table measurement is absurd.

After all, that formula fails to account for the crown and pavilion angle offset. Although that may be true, you probably haven't been buying diamonds as long as the Tooth Fairy. Therefore, we're going to debunk the fairytale and provide some illumination.

Here are some common references for ideal cut diamonds:

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

Consequently, these classifications of diamond cut quality mean nothing by themselves. But they certainly sound impressive, don't they?

The Ultimate Standard for Ideal Diamonds:

How to Use ASET to Judge Black by Brian Gavin Diamond Cut Quality and Performance.

Brian Gavin Black ASET Verifies Consistency.

In the first place, none of the terms above mean anything on their own. In other words, you cannon compare them without a defined set of parameters.

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 of diamonds graded on the proprietary light performance grading platform. That's because the AGSL incorporates 
the Angular Spectrum Evaluation Tool (ASET).

Consequently, Brian Gavin adheres to the AGS Light Performance grading standards. He also holds the patent for maximizing light performance in the modern round brilliant cut diamond.

Defining Ideal Cut Parameters:
James Allen vs Brian Gavin Hearts and Arrows Diamonds.

James Allen True Hearts Diamond Cut Quality.

It's possible to compare diamond proportions with AGS standards. However, the proportions merely set the stage for the performance. In other words, they are only one piece of the puzzle. The degree of optical precision is the final piece that defines diamond cut quality.

Subtle and minute dimensional differences of a degree determine whether a diamond will be brilliant, dull, or listless. The same principle applies to the consistency of facet shape, size, and alignment.

Under those circumstances, there are different classifications of ideal cut. The AGS and GIA cut grades encompass a broad range of proportions. In that case, the Black by Brian Gavin ultra-super ideal cut diamonds are at the top of the threshold.

Diamond Cut Quality By The Numbers:

AGS's original Cut scale has eleven categories of proportions for round brilliant cut diamonds. The highest rating is AGS Ideal-0, and the lowest is AGS-10 Poor. The overall cut grade of the diamond takes the following factors into account:

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

All four categories must meet the criteria for the AGS Ideal-0 rating. Many people produce ideal cut diamonds with varying degrees of precision. However, only Brian Gavin has a patent for maximizing light performance in the modern round brilliant.

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

Ritani Engagement Rings Diamond Cut Quality.

Shop Ritani by Diamond Cut Quality.

In the AGS Laboratory's early days, an AGS Ideal-0 cut diamond was commonly referred to as AGS-000 or Triple Zero. That means that it has zero polish, symmetry, and proportions.

This description is not entirely accurate and can be confusing. That's because the AGSL uses a numerical system of reference. In that case, a true "triple zero diamond" is Flawless, D-color, with an overall cut grade of AGS Ideal-0.

Under those circumstances, anybody who uses the term "AGS-000" to describe anything else doesn't know what they're talking about. Either that or they are several years behind the times and should update their lingo.

Focus on Diamond Cut Quality Not the Description:

Ant 20x magnification.

Real Ant at 20x Magnification.

Regardless of what you call it, a Super Ideal Cut Diamond makes the hairs on a gnats posterior stand up and quiver. Consequently, that's a politically correct colloquialism for people who can't handle the word "ass."

In case you're wondering, we said that to a client once to describe the size of inclusions. Specifically implying that the diamond inclusions are smaller than the hairs on a gnat's ass.

He pressed us for further explanation, but we couldn't catch a gnat without squishing it. So, 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:

The following measurements are necessary to determine diamond proportions accurately:

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

Be aware that Pavilion Angle in Degrees is NOT the same as the Pavilion Depth expressed as a percent. The AGS and GIA gemological laboratories also 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 AGS Cut scale from the mid-1990's consists of eleven categories. Those categories define 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 added crown and pavilion angle to their report.

Consequently, only the AGS Laboratory grades diamonds for light performance. The GIA only takes the proportions into account and does not provide complete 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 their contributions to our industry.

Legacy AGS Ideal Proportions:

Here are the original broad-spectrum parameters for the AGS Ideal proportions rating for round diamonds:

  • 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 Are Too Broad!

We want to emphasize that the proportions range above is too broad by our standards. However, those are the measurements for the original AGS proportions grade.

Consequently, the AGS Ideal-0 rating was only proportions-based until June of 2005. The current Light Performance grading platform is more in-depth.

In fact, it's too complicated to try and explain in a few paragraphs. However, we will make an effort to outline the basics for your foundation of understanding to build upon.

The Superior AGS Light Performance Grading Platform:
How to Use ASET to Judge Black by Brian Gavin Diamond Cut Quality and Performance.

Black by Brian Gavin Diamond Cut Quality.

In the first place, the range of proportions depends on the table diameter. Secondly, the AGSL uses its proprietary Angular Spectrum Evaluation Tool (ASET) to measure light performance.

Interestingly, the Total Depth % measurement of the diamond is not taken into account. That's because the Crown & Pavilion angles dictate light performance. Therefore they represent a more critical factor than total depth.

Consequently, the total depth would normally be acceptable if the crown/pavilion angle offset is in the right range. In that case, the total depth measurement is of no consequence. However, we recommend a total depth between 59 - 61.8% for round diamonds for the reasons explained below.

Is 62% Total Depth Good for Round Diamonds?

Sure, you can stretch the total depth to 62% without negative consequences. However, we wouldn't go higher than that because it's likely to impact performance. It's also likely to affect your perception of carat weight.

It is not uncommon to see AGS Ideal Cut diamonds with total depths as deep as 63.5%. However, we recommend avoiding "steep/deep diamonds" like the plague. In the first place, they're not going to perform well.

Secondly, there is a direct relationship between the total depth and visible outside diameter. The deeper the total depth, the smaller looking your diamond will be for the carat weight.

For example, a 1-carat round brilliant cut diamond has a 6.43 - 6.53 mm average diameter. At least if it has proportions within the range that we recommend. For the sake of comparison, that's about the same size as the eraser on a #2 pencil.

What Happens If the Total Depth is Deeper?
Brian Gavin Diamond Cut Quality Carat Weight Examples.

Examples of Carat Weight by Brian Gavin.

If the total depth is deeper, then the visible outside diameter will be smaller. For example, a 1-carat round diamond with 63.5% total depth might be only 6.3 mm in diameter.

In that case, you're paying a premium for a one-carat diamond and getting the look of a ninety-pointer. Consequently, there's a substantial increase in diamond prices between the 0.99 - 1.00 carat marks.

It's bad business to pay a one-carat price for the look of a ninety-pointer in that event. The concept also holds for diamonds in other weight categories.

Diamond prices increase based upon Price Per Carat (PPC) in specific increments of carat weight. Such as between 0.99 - 1.00 carats, but also in ten-point increments between the range of 1.00 - 1.49 carats.

AGS and GIA Proportions Diagrams Are Drawn To Scale:

Brian Gavin Advanced ASET

Black by Brian Gavin Diamond Cut Quality.

The proportions diagram on the lab report provides the measurements. The AGS's report for this Black by Brian Gavin Diamonds shows the proportions diagram on the left side.

The proportions diagram is drawn to scale depending on the actual measurements. Eight measurements are taken per facet section. The labs add those measurements together and divide them by eight to determine the average.

The AGS Laboratory (AGSL) publishes the average measurement for each facet section. In contrast, the GIA rounds those measurements off further to the nearest half a percent or degree.

The GIA also rounds off the lower girdle and star facet measurements to the nearest five percent. Imagine the effect that can have on your ability to predict GIA-graded diamonds' performance.

Do-do That Voodoo That You Do So Well, GIA:

For the sake of argument, let's say that the average crown angle measurement on the report is 34.5 degrees. That's great if the average results from a tight range like 34.3 - 34.9 degrees.

But what if the spread is between 33.5 - 35.5 degrees? The diamond grading report is going to reflect 34.5 degrees either way. Of course, the same principle applies to the pavilion angle, table diameter, and minor facets.

That is why it's important to analyze the reflector scope images. It's also why we're so excited about Advanced ASET. When you put all the pieces together, you'll be able to buy a diamond with complete confidence.

Tolkowsky Cut and Tolkowsky Range:

Another trick that Ali Baba and his den of thieves relies upon is the infamous tale of Marcel Tolkowsky. He was a physicist who published "Diamond Design" in 1919 as a member of a Belgian diamond cutting family.

That book was the first major analysis of diamond proportions. His work was based on [what was then] modern theories of light behavior. Under those circumstances, Tolkowsky proportions were essential to the advancement of light performance.

Ali Baba likes to say things like, "This diamond has Tolkowsky proportions." However, I didn't want to increase the price by having the diamond ."

This is usually bunk because if the proportions met Tolkowsky's parameters, having it lab-graded would increase the value. Since the cost of sending a diamond to the lab is minimal, not doing so raises doubts.

Consequently, there is no such thing as a certified diamond. According to the fine print on the back of the report, the labs do not certify anything. They only report the characteristics of the diamond at the time of grading.

Thus, we consider the term "certified diamonds" misleading. However, we also recognize the necessity of using the term for SEO since people search for it. Do you see what we did there?

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 diamond's girdle edge be "Knife Edge" or virtually non-existent. That's great in theory; however, such a fragile girdle edge is easily chipped.

Consequently, a knife-edge girdle results in a rating of AGS-10 Poor on the AGS Proportions Scale. Perhaps this is why we have yet to see a diamond with Marcel Tolkowsky's true standards.

Riding on the Coattails of Marcel Tolkowsky:

We see many diamonds purported to be "Tolkowsky Cut" or with those proportions. However, the fine print reveals that the "Tolkowsky range" is within a tolerance of those standards. This is known as the coattail effect, and politicians most commonly use it.

We've evaluated many "Tolkowsky cut diamonds" using computerized proportions analysis by Sarine. Needless to say that these diamonds had more realistic girdle edge measurements.

The proportions rating was equivalent to AGS Ideal-0 in some cases but as low as AGS-2 Very Good in others. In that case, be sure to verify the diamond cut quality and not rely on the name.

We honor the contribution that Marcel Tolkowsky made to the advancement of diamond light performance. However, we no longer live under those lighting conditions. For that reason, we are grateful for ASET and modern technology that makes evaluating diamond cut quality easy.

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 the criteria of their proportions scale.

The old scale appears below as a historical reference because we find it interesting. However, the new proportions scale suggests different 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. Consequently, the average light return of a round brilliant cut diamond with an overall cut grade of AGS-4 good is approximately 70%.

The light return of an AGS Ideal-0 cut round brilliant cut diamond is closer to 96%. That might help you understand why we prefer ideal cut diamonds. Of course, the optical precision of Black by Brian Gavin ultra-super ideal cut diamonds makes them even more impressive.

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.

How to Judge Diamond Cut Quality:

Sunburst Setting by Brilliant Earth Diamonds.

Sunburst Halo Setting by Brilliant Earth.

Unless you're comparing lab-graded diamonds, it might not be easy for you to determine the proportions or diamond cut quality.

Most diamond dealers and jewelry store owners actually don't pay much heed to proportions. They select diamonds 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 has only a basic understanding of the Diamond 4Cs.

A lot of companies on the net rely heavily on this fact. Hence the abundance of virtual diamond inventory with only basic details. Have you noticed how few vendors make it easy for you to see all the measurements?

The Best Places to Search by Diamond Cut Quality:

Thankfully, the crown/pavilion angle measurements are readily available on AGS and GIA diamond grading reports for round diamonds. However, the GIA does not provide those measurements for fancy-shaped diamonds.

Many companies also provide a clarity photograph and video. Fewer provide all the details necessary for you to judge diamond cut quality. Here is a list of our favorite places to shop for diamonds online and what they generally provide:

Naturally, we're here to help you search for the diamond of your dreams. There's even a chance to provide the images necessary to judge diamond cut quality. Sometimes the suppliers provide them within the listing details available to trade members.

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.