GIA vs AGS Diamonds | Choosing the Best Certification

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March 8

People frequently ask about the difference between GIA vs AGS grading standards. In other words, they're trying to determine which diamond grading laboratory is the best.

The differences between GIA vs AGS are something jewelers tend to downplay because GIA's cut grade is proportions-based, and AGSL's is performance-based.

In other words, there are distinct differences between the GIA Excellent and AGS Ideal Light Performance grading platforms. However, the entities similarly grade characteristics such as carat weight, color, clarity, polish, and symmetry.

GIA vs AGS Graded Diamonds from Blue Nile.

Pick Your Favorite Blue Nile Setting.

There are also differences in how the laboratories measure diamonds and calculate proportions. We'll compare GIA vs AGS lab reports for a D-color, Flawless, Black by Brian Gavin diamond to demonstrate this fact.

That should be sufficient proof that the GIA vs AGS grading standards are consistent. At the same time, it will demonstrate the differences between the two gemological laboratories.

GIA vs AGS Grading Standards Side-by-Side:

GIA vs. AGS dual-graded 3.39 carat, D, Flawless, Black by Brian Gavin diamond.

3.39 carat, D, Flawless, Black by Brian Gavin.

AGS vs. GIA dual-graded 3.39 carat, D, Flawless, Black by Brian Gavin diamond.

3.39 carat, D, Flawless, Black by Brian Gavin.

Client Inquiry GIA vs AGS Gem Labs:

Rose Gold Wedding Set by Brian Gavin.

Rose Gold Wedding Set by Brian Gavin.

Here is an email excerpt from a client asking for help selecting diamonds for a three-stone ring:

"Each stone to be round brilliant cut, at least F color, at least VS2, triple excellent for cut/polish/symmetry and fluorescence to not be stronger than medium, and preferably all certified with GIA."

"I haven't really had much to do with AGS, so not sure about that lab. But you seem to hold them in high esteem, so may consider them too." — Ken L.

It might help if I share some insight into the two gemological laboratories.

A Brief History of the GIA vs AGS Laboratories:

GIA vs AGS diamond grading 3-stone ring by Blue Nile.

3-stone Diamond Engagement Ring Blue Nile.

The Gemological Institute of America (GIA) was started in 1931 by Robert M. Shipley. He was a successful retail jeweler who wanted to create a uniform standard of grading diamonds and colored gems. In doing so, he hoped that the public might have greater trust in the industry.

Shortly thereafter, Shipley launched the American Gem Society (AGS) in 1934. The purpose is to unite retail jewelers around a common sense of morals, ethics, and conduct procedures.

The GIA Laboratory issued its first diamond grading report in 1955. It quickly became the industry standard for diamond grading.

Since the GIA was the only premier gemological laboratory for many years, it enjoys a level of market recognition that rivals that of industry giant Tiffany & Co. Still, both have their challenges trying to retain their hold on the market.

Similarities and Differences:

James Allen Petite Pave Setting.

James Allen Petite Pave Setting.

The heavyweight contender for the GIA is the American Gem Society Laboratory (AGSL). The AGS launched the lab in 1996 after the sister organizations suffered a falling out.

Consequently, it was over whether the GIA should add an overall cut grade to their report. The point of contention was whether or not to include the crown and pavilion angle measurements.

I remember thinking how great it would be to have that information added to GIA diamond grading reports. After all, we were selling diamonds that would benefit from the documentation of that type of information. 

Ritani 3-stone Diamond Halo Engagement Ring.

3-stone Halo Setting by Ritani.

However, the majority of the diamond industry flipped out. By that I mean, that they completely lost their minds. After all, they did not specialize in "diamonds of exceptional make," as we called them at the time.

Apparently, they realized that making this type of detailed information available to the public was equivalent to tearing the lid off Pandora's box.


As a matter of fact, they were right, and the battle between the GIA vs. AGS has been going on ever since.

The AGSL Changed the Diamond Industry Forever:

Do You Prefer GIA vs AGS Graded Diamonds?

1.0 and 1.5 carat round diamonds With Clarity.

Peter Yantzer, the first director of the American Gem Society Laboratory, was the lab director for the Gemological Institute of America. I will refer to Peter by his first name in this article because I consider him a friend.

Peter tried to get the board of directors at the GIA to approve an overall cut grade, including the crown/pavilion angle measurements to their diamond grading reports.

As I recall, the proposal was initially met with enthusiasm. However, it wasn't long before some substantial diamond cutting firms threatened to boycott the lab.

They threatened to send all of their diamonds to other, lesser-known laboratories such as the European Gemological Laboratory (EGL) and the International Gemological Institute (IGI).

The Great GIA vs AGS Tea Party:

Under the circumstances above, the GIA Board of Directors rejected the proposal. Even though the talking heads rejected the initial proposal at the GIA, it caught the American Gem Society's board members' interest.

Oval 3-stone Diamond Engagement Ring by Blue Nile.

3-stone Oval Diamond Ring by Blue Nile.

They offered to fund a laboratory for Peter, who promptly opened the American Gem Society Laboratory in 1996. Those who specialized in diamonds of exceptional make found it pretty easy to adopt the new industry standard for diamond grading.

It provided an overall cut grade for round brilliant cut diamonds, using a numerical scale ranging from 0 – 10 based on individual grades for polish, symmetry, and proportions. The diamond's overall cut grade is equivalent to the lowest score for any cut quality parameter.

Another difference between the GIA vs AGS is the information they provide for fancy shape diamonds. The GIA does not provide the crown or pavilion measurements, but the AGS does. Consequently, those measurements are essential for predicting light performance.

Creating the Great Divide:

Black by Brian Gavin Hearts and Arrows Diamonds.

Black by Brian Gavin Hearts and Arrows Diamonds.

The select few of us who specialized in Hearts and Arrows Diamonds and other diamonds of exceptional make loved the cut grade system. However, most of the diamond industry was not eager to adopt the new cut standard introduced in 1996.

We and others like Brian Gavin, James Allen, and Whiteflash focused on light performance. In that case, we were already promoting diamond cut quality with our customers.

Simultaneously, most of the industry was still flying the flag of the GIA Gem Trade Laboratory.

Obviously, they were hoping to sink the S.S. AGSL by either flat out denying the laboratory's existence or challenging the validity of their grading practices altogether.

3.50 carat, E-color, VVS-1 clarity, Black by Brian Gavin Diamond.

Sizzling Sparkle of Black by Brian Gavin Diamonds.

Believe me; nothing was more entertaining at the time than one of our customers calling us up to tell us that they had just walked out of a jewelry store that promoted themselves as a Member of the American Gem Society, but who purported only to sell diamonds graded by the GIA.

The reason given is "because they are more accurate," and apparently, they weren't aware that their "society" had launched a gemological laboratory! Whoops, it might be time to crack open a trade magazine or read your mail.

The stories became funnier when our clients told us that the "diamond experts" at the store didn't know the crown and pavilion angle measurements for the GIA diamonds they were selling.

The only response that they seemed to have for any questions about "Hearts and Arrows Diamonds" was that it's all smoke and mirrors. Yet, they couldn't provide any insight into how the magic trick was performed (because they'd never seen one).

Awakening the Sleeping Giant:

Brilliant Earth Cushion and Round 3-stone Engagement Ring.

Brilliant Earth Cushion & Round 3-Stone Ring.

Now June of 2005 is a bit of a blur to me because my wife Robin committed suicide at the beginning of the month. Yea, sorry, I didn't see that one coming either.

If you want to know more about it, you can read my blog post "The Suicide Note." I wrote it on the seventh anniversary of her death in response to her note.

One of the things that I distinctly remember about that time in my life is being amused by the GIA proudly announcing that they were adding a proportion-based cut grade.

Consequently, the AGSL simultaneously announced that they were moving beyond proportions-based cut grading. Their new focus was a grading platform that revolves around the diamonds' actual light performance.

It Took the GIA 10-years to Catch-up:
Black by Brian Gavin Diamond.

Black by Brian Gavin Diamonds ASET.

I remember thinking that it took almost a decade for the GIA to acknowledge the importance of cut grade and the effect that the crown and pavilion angles have upon light return.

Then the AGSL rolls up alongside them and literally blows the GIA out of the water by introducing their Angular Spectrum Evaluation Tool (ASET).

In doing so, the AGS moved the industry forward light-years beyond the proportions-based cut grade system. The GIA still employs almost two decades later.

In the battle between the GIA vs. AGS, the AGS Laboratory is clearly in the lead. And we haven't yet discussed the recent introduction of Advanced ASET. Nor have we talked about the patent for the Black by Brian Gavin Diamonds.

Is the GIA or AGS More Accurate?

Tapered Baguette 3-stone diamond engagement ring by James Allen.

Tapered Baguette 3-stone Ring by James Allen.

I've had my fair share of experience sending diamonds to both the GIA and the AGS gemological laboratories. There are all sorts of theories about how each laboratory grades diamonds differently and that one is more accurate than the other.

I have to tell you; I find them to be comparable in the level of consistency for carat weight, color, clarity, polish, symmetry, proportions, and fluorescence.

I've sent diamonds to both laboratories for a side-by-side comparison of the diamond grading reports. Based on that experience, I feel that they are comparable in their grading standards.

As a matter of fact, this is not really surprising since both laboratories employ GIA Graduate Gemologists as diamond graders. Therefore the GIA is still setting the standards for diamond grading since they teach people how to grade diamonds.

Measuring Diamonds by GIA vs. AGS Standards:

Keep in mind that the proportions of a diamond are determined by measuring the diamond using computerized proportions analysis. As a matter of fact, both the GIA and AGS use Sarin machines to measure their diamonds.

However, each one uses proprietary software that Sarin has designed to suit their individual grading standards for proportions. There is a minimal variance between the gemological laboratories concerning how diamonds are measured under those circumstances.

Factors such as clarity, color, and fluorescence are graded by human eyes, subject to various conditions. Consequently, that includes the individual vision of each diamond grader.

Each gemological laboratory also has its own system for determining clarity, color, and fluorescence. As a matter of fact, it is quite common to have more than one grader look at a stone.

I have yet to see a discernible difference between the GIA Excellent and AGS Ideal grades for polish and symmetry. With that in mind, I consider GIA Excellent to be the equivalent of AGS Ideal for polish and symmetry.

GIA Excellent vs. AGS Ideal Cut:

Choosing between GIA vs AGS Graded Blue Nile Diamonds

Everybody Loves the Sparkle of Blue Nile Diamonds.

The biggest difference between the GIA Excellent or "GIA 3X" overall cut grade and the overall cut grade of AGS Ideal-0 lies in the differences between the parameters for the GIA Excellent proportions and the AGS Ideal proportions grading platforms.

It seems to me that the parameters for the AGS Ideal-0 proportions grade are a lot tighter than the guidelines for the GIA Excellent proportions grade.

To the extent that I often find myself staring at the "Cut Grade Excellent" line on some GIA diamond grading reports and the proportions outlined on the proportions diagram and wondering how they made that determination.

Admittedly, I'm afraid I also have to disagree with some of the proportion combinations which can obtain an AGS Ideal-0 proportions grade. I'm specifically referring to those on the outlying regions of the proportions charts used by the AGSL.

Is GIA Excellent the Same as AGS Ideal Cut Proportions?

GIA Excellent cut round diamond from James Allen.

James Allen GIA Excellent Cut Round Diamond.

Not surprisingly, the difference between the proportions specifications for AGS Ideal and GIA Excellent are not crystal clear. For instance, take the measurements of this 1.67 carat, F-color, VS-2 clarity, GIA 3X diamond from James Allen, pictured here.

It has a total depth of 61.4% with a table diameter of 56% and a crown angle of 35.5 degrees offset by a pavilion angle of 41.0 degrees with a thin to medium, faceted girdle.

The overall cut grade is GIA Excellent (proportions, polish, symmetry). I can tell you from experience that the combination of the 43.5% pavilion depth is a problem. It happens to be the critical tipping point where I find that light begins not to strike fully off the pavilion facets.

The 85% lower girdle facets effect will also produce sparkle that is extremely small in size. That will affect our human eyes' ability (instead of a camera lens) to disperse white light's flashes into colored sparkle/fire.

With that in mind, this diamond is likely to perform best in pin-fire type lighting conditions, e.g., candlelight and halogen jewelry store lighting, but not so well under normal lighting.

AGSL Proportions Chart for 56% Table Diameter:

GIA vs. AGS Graded Diamonds, Legacy AGSL Proportions Chart for Round Diamonds 56% Table Diameter

Legacy AGSL Proportions Chart.

Here is a portion of the proportions grading chart published by the American Gem Society Laboratory in 2008 for round brilliant cut diamonds.

To use it, go to the diamond grading chart for the diamond's table diameter, which is 56% in this case. Then cross-reference the measurements for crown and pavilion angle.

In this particular instance, the diamond's crown angle is 35.5 degrees, and the pavilion angle is 41.0 degrees. I cropped down the image on the right and bottom sides.

The cross-section where the two measurements meet on the chart is the red box that appears in the lower right corner of the graph, as indicated by the red arrow.

On the scale of the AGSL proportions published in 2008, the color pink is used to indicate AGS Ideal-0 proportions. The color red represents AGS-1 Excellent.

Gold indicates AGS-2 Very Good; and the color yellow indicates AGS-3 Good. So the proportions rating of this diamond would be AGS-1 Excellent.

With that in mind, in this particular bout of the GIA vs. AGS, the overall cut grade of GIA Excellent is not equivalent to the AGS Ideal-0 cut rating.

However, keep in mind that this does not take the crown height or pavilion depth measurements into account, and thus it reveals only part of the picture!

How to Interpret AGS ASET:

Advanced ASET Brian Gavin Blue Fluorescent Diamond.

Advanced ASET for Brian Gavin Diamonds.

The differences between the AGS and GIA gemological laboratories' highest overall cut grades do not end with their proportions grade parameters.

The biggest difference between the GIA vs. AGS is that the AGSL employs their Angular Spectrum Evaluation Tool (ASET) on their Proprietary Light Performance grading platform.

The diamond grading report for this 2.10 carat, D-color, VS-1 clarity, Brian Gavin Blue fluorescent diamond, features Advanced ASET. The ASET map uses the colors red, green, and blue to represent brightness and contrast.

All of that red that is visible in the image represents the brightness of the diamond. The light green color indicates less bright areas, and the blue arrows show the main pavilion facets reflecting the camera lens.

Notice how symmetrical the patterns of light reflection are within this Black by Brian Gavin diamond. The symmetry of the patterns represented by each color indicates the precision of optical symmetry that this diamond has been cut to, directly affecting the sparkle factor exhibited by this diamond.

The higher the degree of optical symmetry, the more contrast brilliance you'll see within the diamond. Thus the more vivid it will appear, especially since the lower girdle facets range from 75 – 78%. That will produce larger sparkle that is bolder, brighter, and more vivid!

Should the Center of an ASET Be Red or Green?

GIA vs AGS Graded Diamonds ASET Distinguishes the Difference.

ASET Scope Image for Brian Gavin Diamond.

Before you get too caught up in all of those red and green sections portrayed on the ASET image, I should point out that there will be many times when the two colors will meet or blend in a particular section of the diamond.

That is because the edge of the range for both red and green are shared by both colors when the diamond is gathering light from within the room from the vantage point of 45-degrees.

Don't freak out if you see both red and green somewhere like the center of the table facet, facing up as all green in this particular image because it's perfectly normal. In other words, the middle of an ASET Scope image can be red or green, and it doesn't matter which.

I Prefer the AGS Lab (it's no secret):

All right, I have nothing but respect for the GIA and their Gem Trade Laboratory, their contribution to the diamond industry is undeniable, and I find their grading practices to be consistent.

However, I personally prefer the additional insight provided by the ASET results on the AGSL Proprietary Light Performance Diamond Quality Document. Assuming that with all other factors being essentially equal between the GIA vs. AGS diamonds you are considering.

For example, if I were considering two diamonds, one graded by the GIA and the other by the AGSL, with similar characteristics. Meaning that they have the same carat weight, clarity, color, fluorescence, polish, symmetry, and proportions.

Additional information, such as the ASET image and other reflector scope images, such as the Ideal Scope and Hearts & Arrows Scope images, enable me to take the diamond's optical symmetry into account.

GIA vs. AGS Grade Comparably, but...

Unfortunately, most vendors are not equipped to provide these images for their customers. So, it makes my job easier if the Diamond Quality Document (DQD) provides the ASET map.

However, this does not mean that you should only consider diamonds graded by the AGSL. After all, there are many exceptional GIA diamonds with an overall cut grade of GIA Excellent. By the way, that is commonly referred to as "GIA 3X" at the industry level.

When looking for round brilliant cut diamonds, I recommend limiting your search to diamonds with the following characteristics. In the first place, search only for diamonds with an overall cut grade of AGS Ideal-0 or GIA Excellent. Then look for the following range of proportions:

  • Total depth between 59 – 61.8%.
  • Table diameter between 53 – 58%.
  • Crown angle between 34.3 – 35 degrees.
  • Pavilion angle between 40.6 – 40.9 degrees.
  • Lower girdle facet length between 75 - 80%.
  • Girdle between 0.7% thin to slightly thick.
  • Culet: GIA "none” or AGS "pointed” (same thing).
  • Polish of AGS Ideal or GIA Excellent.
  • Symmetry of AGS Ideal or GIA Excellent.

Now Here's the Twist:

Oval, Round, Marquise-shaped Diamond Engagement Rings James Allen.

Petite Pave Setting by James Allen.

The parameters that I outlined above represent the middle of the spectrum, or "the sweet spot" within the range that the AGSL specified for their zero ideal cut proportions rating.

However, other combinations of crown and pavilion angles will provide a comparable volume of light return. Not really, but saying that seems to appease the people who prefer a broader range of proportions.

Rather than get into a really long dialogue about all of the possibilities, feel free to consider me your Personal Diamond Shopper. Please send me the details of any diamonds that you're considering. I'm happy to look over the details for you and make recommendations.

About the Author

Dive deep into the glittering world of diamonds with Todd Gray, the CEO of Gray Matter Development, LLC. Todd has 35+ years of experience as a diamond buyer and trade consultant. He ghostwrites content for several online vendors and is an avid Freediver, currently exploring the Cenotes of Yucatan, Mexico. Dive into brilliance with Todd Gray!