Diamond Cut Grades

The Old System :: Guessing Based on Measurements

The beauty of a diamond is affected more by the skills of the cutter than by anything else. The cut is more important than clarity, color, or carat weight in the way of producing beauty in a diamond.

But… until January 1, 2006, it was also the hardest aspect of diamond quality to assess and compare, especially as a consumer. Until then, your only choice for assessing cut quality was to use a proportion-based approach. This required you to ::

  1. assume there is one and only one ideal set of proportions, at that time using the Tolkowsky mathematical proportion set
  2. assume that a diamond of those proportions was ideal, and anything that varied from that was less than ideal
  3. study study study how each of the various measurements and proportions affected performance and beauty
  4. compare all diamonds based on those assumptions and all of your study
Using proportions, it used to be necessary to guess on the beauty of a diamond, based on assumptions about certain measurements, angles, finish, polish, symmetry, culet size, and so many other factors.

The proportion based method never gave us the actual results. It only predicted, and not always accurately.

It also left out many alternative proportion sets that have since been discovered to perform just as well as the so-called ideal proportion set.

This is the market I grew up in, back in the 80′s, when I bucked the system and started the trend to educate consumers against the will of the retail industry, where the practice was to keep it all a mystery and keep the secrets and profits to themselves. That never felt right to me.

The New System :: Direct Assessment of Light Performance

But then, in January of 2006…

GIA introduced a system based on the actual measurement of the performance, rather than assumptions about the proportions.

They used scientific observation of millions of round diamonds of all proportions, both through direct observation by consumers and experts, and through mathematical modeling of every combination and permutation of the variables, and discovered something entirely new…

… a performance-based system that does NOT assume one perfect set of proportions, but instead that awards excellence to any combination that performs at the highest level.

It was discovered that many combinations produced light performance at the highest level, to match or in some ways exceed the Tolkowsky Ideal. It is true that the Tolkowsky proportions were one of the proportion sets that performed the best, but there were others, and the new system finally awarded those stones the grade they deserve.

In other words, this new performance-based cut grading system is based on measuring actual light performance, and not just assuming light performance based on someone’s preferred proportions.

This makes it unnecessary to study all the minute details (and they are infinite) of the measurements and proportions of a diamond of any shape.

Since then, with the introduction of Cut Grades by GIA, AGS, EGL, and many other labs, the consumer can much more easily compare diamonds for beauty and make a more intelligent choice.

This is HUGE for consumers! It simplifies the most difficult part of diamond shopping. 

I will teach about the two major approaches, backed by the strongest and most extensive research, the GIA and AGS systems for diamond cut grades.

As a result, I no longer teach all the ins and outs, with all the miniscule guidelines about every little detail.

Just keep in mind, that you can NOT learn all there is to learn about diamond cutting and proportions and light performance without a lifetime of study. Even I do not know it all, and I have been in this business since the 80′s. But with the Cut Grade idea, as it was with Color and Clarity in the 50s, we have a tool to compare diamonds and you do not need to know it all yourself. Yeehah!

GIA Cut Grade System

The grading nomenclature is simple ::

  • Excellent
  • Very Good
  • Good
  • Fair
  • Poor

The definitions for the grades are not so simple. Each grade includes analysis of many bits of data, including brightness, fire, leakage, contrast, weight ratio, girdle thickness, durability, tilt, culet, polish, symmetry, and other factors.

But basically I would define the five grades in this way, for the purpose of buying a diamond engagement ring.

My Non-Technical Descriptions for Cut Grades ::

I know these descriptions of the Cut Grades will not sound scientfiic, but this is a purchase of emotion, and not a science project. The links below to the GIA site will supply the science in its full glory. I just want to help you buy a diamond with the greatest beauty for your budget.

Excellent Cut Grade refers to a diamond that is going to elicit the best response, when compared side by side with any lesser diamonds, as the best on the table, very consistently. There is a slight upgrade when you compare Excellent and Very Good.

(NOTE :: These diamonds will often but not always produce the Hearts and Arrows effect that can result from extreme precision in symmetry and proportions under specialized viewers. The highest cut grade by AGS will always produce some degree of these effects by my experience. H&A is not the goal of GIA, but rather brilliance and overall beauty.)

Very Good Cut Grade refers to a diamond that has great beauty when seen by itself, and no improvements will come to mind. The diamond will impress anyone, and will hold up well in any light for almost any environment. The only way you could see a difference with an Excellent Cut Grade is to see it side by side, and it will still be subtle for most people.

Good Cut Grade refers to a diamond that has adequate light performance and beauty to be appealing to most people. Diamond, after all, has exceptional natural properties for becoming brilliant, scintillating, and firey. Good cut means the diamond has achieved it’s goal of beauty, but not to the fullest degree possible.

Fair Cut Grade refers to a diamond that will not compare well side by side with most other diamonds in the marketplace today.

Poor Cut Grade is not a diamond to own in my opinion, and neither is a Fair. Either of these should be avoided these days, since so many other choices of greater beauty are easily and readily available.

NOTE :: Currently GIA offers cut grades for Round Brilliant Cut diamonds only.

More Information ::

Read here in great detail about the GIA Cut Grade system.

Here is the page for GIA’s introduction to the GIA Cut Grade and how they explain it.

Here is their interactive page on Diamond Anatomy.

Here is the GIA Facetware Cut Estimator, which is actually proportion based, so you cannot rely on the accuracy entirely. Its just a tool to give you a little fun perspective about cut grades. The diamond you send to their lab might not get the same grade as from the calculator. The numbers are not the whole story, but they give an idea.

My Advice ::

For those of you who like perfection, then go for GIA Excellent Cut Grade. For those of you who like something that looks great but saves a little money, go for GIA Very Good Cut Grade.

Nothing more to it.

AGS Cut Grade System

More about the AGS Cut Grade System.
(I was at the first presentation of the full data, before the grades were offered on their lab reports, at the JCK National Jeweler’s Convention in Las Vegas, in 2005.)

NOTE :: Currently AGS offers cut grades for Round Brilliant Cut, Princess Cut, Oval CutEmerald Cut and Cushion Cut shapes.

The AGS Laboratories uses the AGS Performance-Based Cut Grade system, which is based on the overall performance of a diamond, specifically in the area of Light Performance. When analyzing the overall performance of the diamond, they take the following into consideration: light performance (contrast, leakage, dispersion, and brightness), proportion factors (girdle thickness, Culet size, Weight Ratio, durability, and tilt), and finish (Polish and Symmetry).

Here is the chart of grades from the AGS website ::

Screen Shot 2013-12-06 at 5.51.46 PM

My Descriptions ::

Ideal Cut Grade – refers to a diamond that is going to elicit the best response, when compared side by side with any lesser diamonds, as the best on the table, very consistently. There is a slight upgrade when you compare Excellent and Very Good.

(NOTE :: These diamonds in my experience will always produce some degree of the Hearts and Arrows effect that can result from extreme precision in symmetry and proportions under specialized viewers. The highest cut grade by GIA will not always produce some degree of these effects from what I have seen.)

Excellent Cut Grade – refers to a diamond that has great beauty when seen by itself, and no improvements will come to mind. The diamond will impress anyone, and will hold up well in any light for almost any environment. The only way you could see a difference with an Excellent Cut Grade is to see it side by side, and it will still be subtle for most people.

Very Good Cut Grade – refers to a diamond that has adequate light performance and beauty to be appealing to most people. Diamond, after all, has exceptional natural properties for becoming brilliant, scintillating, and firey. Good cut means the diamond has achieved it’s goal of beauty, but not to the fullest degree possible.

Good Cut Grade – refers to a diamond that will not compare well side by side with most other diamonds in the marketplace today.

Fair and Poor Cut Grade - is not a diamond to own in my opinion, and neither is a Fair. Either of these should be avoided these days, since so many other choices of greater beauty are easily and readily available.

My Advice ::

If you want perfection and beyond, into the story and curious symmetry of the Hearts and Arrows patterns under specialized viewers, then get the AGS report and insist on the AGS Ideal Cut Grade. The report will include a photo of the patterns under the special viewers.

If you want perfection to the eye, the AGS Excellent Cut Grade is exactly your cup of tea.

If you want beauty, and a positive reaction from anyone, but want to save a little money, then go with AGS Very Good.

I would personally stay away from anything else, but I am a bit of a diamond snob.

NEXT :: Diamond Carat Weight

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