The AGS vs GIA gemological laboratories are the industry standard for diamond grading. The two labs grade carat weight, color, and clarity similarly. However, there are differences in how they measure diamonds and calculate proportions.
There are also differences in the light sources they use to grade fluorescence. However, the basics of polish and symmetry grading are the same.
Consequently, the abbreviation for the American Gem Society Laboratory is AGSL. The abbreviation for the Gemological Institute of America Gem Trade Laboratory is GIA-GTL, but most people shorten it to GIA.
Update: the AGS Laboratory is closing at the end of 2022.
AGS vs GIA Diamond Grading Reports:
Both the AGSL and the GIA are Heavy Weight Contenders which command the highest respect throughout the industry. As a matter of fact, they are extremely well-matched in terms of the grading standards that they use to grade diamonds for:
However, there are distinct differences in how they approach grading diamond cut quality. As a matter of fact, that is the defining characteristic that distinguishes the AGS from the GIA.
History of the AGS vs GIA:
The Gemological Institute of America (GIA) was founded in 1931 by Robert M. Shipley. As a matter of fact, Shipley was named Man of the Century by JCK Magazine in December of 1999.
I wholeheartedly agree with JCK's award of this prestigious title regarding his contribution to the gemological community. After all, it is difficult to imagine the diamond and gemological business without thinking of ways that it has been affected by Robert Shipley and the Gemological Institute of America. Photo courtesy of the Gemological Institute of America.
As stated in the article, "Consider his accomplishments: He introduced gemological training to the United States and established the first gemological school, the first research laboratory, and the first professional gemology journal in North America.
Shipley pioneered the creation and use of gemological instruments. He also founded the first professional society of jewelers in North America and organized the first national educational conclaves of jeweler/gemologists.
As a matter of fact, he also instituted industry-wide standards for grading and nomenclature. As a matter of fact, he even helped create the "four Cs" of diamond buying."
The American Gem Society:
By the way, the preceding reference to "the first professional society of jewelers in North America" refers to the American Gem Society, created by Shipley in 1934.
At that time, the GIA intended to provide jewelers in North America with gemological education. In contrast, the AGS serves to provide a foundation of morals and ethics for responsible jewelers to follow.
According to the article by JCK Magazine, "Those GIA courses and the early AGS conclaves stitched together a national community of jeweler-gemologists, unifying the industry at the grassroots."
When I first got started in the diamond business back in October of 1985, it was explained that:
"The Gemological Institute of America and the American Gem Society were "sister organizations" founded by Robert M. Shipley to create a uniform grading system for diamonds and colored gems used worldwide and a foundation of ethics for the industry to follow."
Needless to say that my hat goes off to Robert M. Shipley because he was a major contributor to the trade that I enjoy. At the same time, I use the information taught to me by the GIA almost every day… so thank you.
Sometimes Sisters Squabble:
Especially when money (or boys) are involved.
Everything seemed to be going fine between the two sister organizations until the mid-1990s. That's when Peter Yantzer, Director of the GIA Laboratory, suggested to the Board of Directors that additional criteria be added to the lab report format.
At that time, the diamond grading reports issued by the Gemological Institute of America Gem Trade Laboratory (GIA-GTL) were missing some critical components.
Specifically, the crown and pavilion measurements represent the top and bottom halves of a diamond. As I recall, the GIA Board of Directors initially accepted the addition of the crown and pavilion measurements.
However, they later rejected the proposal due to pressure from various diamond cutters. Apparently, they threatened to boycott the laboratory if they insisted on providing the public with that sort of detail.
The Shadow Conspiracy:
Now I'm not suggesting that certain diamond business elements prefer to keep the diamond buying public in the dark. Thereby keeping them guessing about the details of diamond cut quality… No, I'm just going to come right out and say it.
The majority of the diamond industry seems to prefer customers who are not brilliant, pun intended. Keep in mind that all of this was occurring when the world was beginning to wrap its mind around the power of the internet.
As a matter of fact, most of the diamond industry was losing its mind. In fact, more than fifty members of a diamond trade network known as Polygon filed a lawsuit against us DBA Nice Ice and Treasures by R.J. for "the disclosure of proprietary information to the public" and "disparagement of an entire industry."
Hey, I'm rather proud of myself for that last part, "the entire industry" = impressive. If only I could find a picture of a dog licking its butt.
Because then I could show you what those fifty jewelers looked after the Law Firm of Orrick, Herrington & Sutcliffe spanked them. How do you spell R-E-T-R-E-A-T?!?!
You've got to love the First Amendment. After all, it provides us with the protection of free speech, freedom of expression, and specific protection for satirical wit. Ahem.
Original GIA Lab Report Format:
The original diamond grading report format issued by the Gemological Institute of America Gem Trade Laboratory provided fundamental details about the proportions of a diamond:
The outside diameter and depth measurements, along with the total depth, table diameter measurements, girdle thickness, and culet size.
The GIA did not originally provide critical data such as the crown angle, crown height, pavilion angle, and pavilion depth measurements.
Under those circumstances, there is no way for you to determine how the carat weight is distributed between the upper and lower halves of the diamond. Needless to say that this "diamond grading report" format left a lot to be desired.
As a matter of fact, I'm not even sure that it should have been referred to as a report since critical data was omitted, intentionally or not.
At some point, the Board of Directors for the American Gem Society elected to offer Peter Yantzer the opportunity and funding to open a gemological laboratory at their facility in Las Vegas, Nevada. The rest is history. The American Gem Society Laboratory opened its doors in 1996.
AGS vs GIA Grading - Which Is More Accurate?
People frequently ask me whether the American Gem Society Laboratory grades as consistently as the GIA Gem Trade Laboratory. Whether the AGSL is as strict as the GIA.
Whether the GIA is stricter on color or clarity grading than the AGSL. Whether the AGSL is less strict on color or clarity grading. Which diamond grading laboratory do I prefer, etc., and the answer always seems to surprise them.
Are you sitting down? Because here it comes.
AGS DQD for Brian Gavin Signature diamond:
The American Gem Society Laboratory issued the Diamond Quality Document below on December 09, 2013. Obviously, this 1.255 carat, D-color, Internally Flawless clarity, Brian Gavin Signature round hearts and arrows cut diamond has since sold:
Here are the specifications of this Brian Gavin Signature round hearts and arrows cut diamond. According to the AGS Laboratory, it measures 6.88 - 6.90 x 4.27 mm.
It also has a total depth of 61.9% and a 56.7% table diameter. The pavilion angle is 40.7 degrees and is offset by a crown angle of 35-degrees. Consequently, the pavilion depth of 43% and the crown height of 15.2% are correct for those angles.
The diamond features 76% lower girdle facets (LGF) and star facets that measure 53% with a thin to medium, faceted girdle and a pointed culet. As a matter of fact, you will see that the measurements as provided by the GIA below are quite similar.
However, there will be slight differences due to how the labs determine the measurements and round them off on the reports. Consequently, I prefer how the AGS presents the measurements.
AGS vs GIA Grading (for the same diamond):
As it so happens, Brian Gavin submitted the same diamond to the GIA for grading on February 27, 2015. For those of you keeping track, that is a little more than two years later.
At the time, the GIA graded the same BGD Signature diamond as follows: 1.25 carats, D-color, Internally Flawless clarity, with an overall cut grade of GIA Excellent. The GIA diamond grading report number is 2145936424, and you can verify the details via GIA Report Check.
According to the GIA, the diamond measures 6.89 - 6.91 x 4.27 mm. The diamond has a total depth is 61.9% with a 57% table diameter. The pavilion angle is 40.8 degrees offset by a 35-degree crown angle.
They also indicate that the lower girdle facet length is 75% with 55% star facets. And the diamond has a medium to slightly thick, faceted girdle.
AGS vs GIA Proportions Guidelines:
In this case, the same diamond was submitted by Brian Gavin to the two laboratories for dual-grading. Consequently, that makes it possible for us to see the differences and similarities in their grading standards.
Here is a comparison of the characteristics as determined by the AGS vs GIA laboratories:
AGS vs GIA Grading Results Ideal vs Excellent:
Comparing AGS vs GIA Details for the Same Brian Gavin Signature Diamond.
Lab Report #:
December 13, 2013
February 27, 2015
Overall Cut Grade:
6.88 - 6.90 x 4.27 mm
6.89 - 6.91 x 4.27 mm
Lower Girdle Facets:
Star Facet Length:
Some of These Things Are Not Like the Other:
As you can see, there are slight differences between the format of the data provided by the AGSL and the GIA. While the measurements are similar, they are also dramatically different.
In that case, you might wonder why the measurements so different if the diamond is the same. As a matter of fact, the answer is straightforward; the GIA rounds off the measurements to the nearest half a percent or half a degree.
Whereas the AGSL reports the actual measurements without rounding them off. Under those circumstances, the crown height of 15.2% reported by the AGSL is rounded to 15% by the GIA.
Similarly, the lower girdle facet length of 76% is rounded down to 75% because it falls below 77.4%. Consequently, it would have been rounded up to 80% if it were 77.5% or higher.
AGS vs GIA Methods of Measuring Diamonds:
The practice of rounding off measurement by the GIA makes it practically impossible to estimate visual performance. At least based upon the data provided on a GIA diamond grading report.
After all, the measurements have been rounded off to the point where they are practically useless.
Take the lower girdle facet length, for example; there is a variance of approximately 2.5% in either direction of the stated value. Similarly, notice how the AGS indicates that the star length is 53%.
Whereas the GIA rounded that measurement up to 55%, which is a 2% difference. Consequently, if it had measured 52.4% instead of 53%, the GIA would have rounded it down to 50%.
In that case, the star facet length measurement also has a potential variance of as much as 2.5%. Under those conditions, the GIA's proportions are practically useless for estimating light performance.
Obviously, I'm Poking the Bear:
Of course, some people will tell you that this variance is acceptable because it represents the average proportions. In other words, the labs take eight measurements per facet section and then add those numbers up and divide by eight.
Although that is technically true, those people miss the point because both the AGS vs. GIA uses computerized proportions analysis machines to measure the diamonds they grade.
Then, those machines report the average of the eight measurements taken per section. As a matter of fact, those are the measurements indicated on a Diamond Quality Document (DQD).
However, the GIA takes those measurements and rounds them off further. In some cases, they round the measurement off to the nearest half a degree or percent.
Then, they round the measurements off to the nearest 5% in other places! Five percent! Just take a moment and let that sink into your mind. Then tell me whether you prefer the grading standards of the AGS vs. GIA.
AGS vs GIA Methods for Measuring LGF:
Another thing to be aware of is that the AGS vs. GIA measures the lower facets differently. As a matter of fact, they measure the LGF using dramatically different methods!
The AGS measures lower girdle length by height, as shown on the left. Consequently, this is also how the DiamCalc software designed by the University of Moscow does it.
While the GIA measures lower girdle length by radius because independent gemologists can measure it using a table gauge if the diamond is mounted. Or if they do not have access to computerized proportions analysis.
The difference between how the AGSL and GIA-GTL determine the lower girdle facet measurements can create confusion. After all, people tend to assume that the measurements are determined the same way.
When actually the difference in how they are measured looks more like the representation on the left. Do you have a headache yet?
Because I wouldn't be surprised if you do, unfortunately, it might get a little bit worse before we're finished. Although that may be true, at least you'll have a better understanding of why I prefer the AGS vs. GIA.
LGF Conversion Chart AGS vs GIA:
Get a load of these charts created by Bruce Harding and Jason Quick of the American Gem Society Laboratory. They show the relationship between the GIA Lower Girdle Radius and AGS Lower Girdle Height measurements. Consequently, the AGS method of measuring the LGF is also used by DiamCalc:
Pay particular attention to the measurements between the range of 75 – 80%. Consequently, that is the same range we recommend in the 1-minute diamond buying guide. Once again, it's clear why I prefer the Light Performance grading platform of the AGS Laboratory.
How to Judge Light Performance & Optical Precision:
Note that the proportions of a diamond are only one piece of the puzzle. Reflector scope images like those Brian Gavin provides for his Signature diamonds are necessary to judge the degree of optical precision that the diamond has been cut to.
Optical precision refers to the consistency of facet size, shape, and the alignment or indexing of the facets polished onto the diamond's surface from the perspective of 360-degree alignment.
Here are the reflector scope images for the 1.255 carat, D-color, Internally Flawless, Brian Gavin Signature diamond that we've been discussing:
This series of reflector scope images provided for the 1.255 carat, D-color, Internally Flawless, Brian Gavin Signature diamond. As a matter of fact, these images are available for all Brian Gavin Signature round diamonds. The images provided in order from right to left, and their purpose is as follows:
Why AGS vs GIA Is Better for Light Performance:
Only the American Gem Society Laboratory (AGSL) uses Angular Spectrum Evaluation Technology (ASET) to measure the brightness of diamonds and determine how effectively a diamond is making use of the light available to it from within the room.
The ASET scan results are provided on AGS Light Performance Diamond Quality documents and responsible vendors. Brian Gavin Diamonds and Victor Canera provide them on the diamond details pages for hearts and arrows cut diamonds.
These three vendors also routinely provide Ideal Scope images for their round brilliant hearts and arrows cut diamonds, along with most other diamond shapes they sell.
We use the Ideal Scope to determine the extent to which a diamond is leaking light due to the combined effect created by the diamond's proportions and facet structure.
These three vendors also specialize in Hearts and Arrows quality diamonds, graded by the American Gem Society Laboratory with an overall cut grade of AGS Ideal-0.
Thus they provide Hearts & Arrows scope images for their hearts and arrows diamonds so that you can verify that they have been cut to the highest degree of optical precision possible.
The details pages for James Allen True Hearts diamonds feature an image of the hearts' pattern and an Ideal Scope image. Still, they do not provide an ASET image, and the diamonds are graded by either the AGSL or GIA gemological laboratories.
I've noticed that many of them are graded by the AGSL on the grading platform that does not provide an ASET image, which seems a bit suspect to me since it is a step down from the ASET-based Light Performance grading platform.
Scratching Out the Differences of AGS vs GIA Reports:
I invite you to take advantage of my free Diamond Concierge Service if you would like assistance finding the diamond of your dreams.
We're also happy to help look over the details for a diamond that you may be considering. As a matter of fact, it doesn't matter whether the diamond is offered online or from your local retail jeweler.
Just send me the details about the diamond you are looking for. Or, send me a link to the diamond details page or the AGS vs. GIA diamond grading report number. That way we'll be able to look up the details and provide you with our thoughts.
If you want my help searching for a diamond, please provide me with the shape of the diamond you seek, along with the range of carat weight, color, fluorescence, clarity, and price that you are working with. Thank you.