This post was written before the GIA and AGSL merged in 2022. Even though the merger has occurred, the information contained within is still of great value to anyone who owns or may potentially acquire a diamond that was graded by AGSL prior to the merger.

The AGS vs. The GIA Diamond Grading Laboratory

When it comes to investing in a diamond, it’s important that you truly understand what you’re buying. After all, how else will you know what you’re getting for your money?

There is a whole host of grading labs around the world. However, it’s important to recognize that each lab uses its own grading system, meaning that a report from one lab may show different results to one issued by a different lab, even if they are both in reference to the same diamond.

AGS and GIA are two of the most respected labs in the industry, having both come up with diamond certifications grading systems that grade diamonds according to their clarity, cut, color and carat.

Although there are other labs out there, such as the EGL for example, I would only ever recommend buying a diamond with a genuine AGS or GIA grading.

So let’s take a look at how AGS and GIA, arguably the two most important gem labs in the world, compare…

GIA – Gemological Institute of America

The Gemological Institute of America is the world’s leading gemological laboratory and the world’s most trusted and widely used service. Widely respected across every continent, the GIA was founded in 1931 and became the first institute to create the 4Cs and the International Diamond Grading System – a standard that is still used today, nearly 80 years later.

In addition to its head quarters in Carlsbad, California, GIA has facilities in most of the major trading centers in the world, employing more than 1400 scientists, educators, and certified diamond graders.

Testament to its commitment to delivering quality and excellence, GIA also offers a range of educational services and carries out intensive research work in order to at the forefront of fast changing technologies in the gemological field.

AGS – American Gem Society Laboratories

The American Gem Society is a US-based laboratory founded in 1996, renowned for its scientific approach and forward thinking research into diamond cut grading. Since then it has made several breakthroughs and has helped pave the way for what consumers should expect from the cut of their diamonds.

Founded by Robert M Shipley who, interestingly, also founded the GIA, AGS wasn’t ever intended to be a lab when it first opened and so wasn’t established as a competitor to GIA. In fact, AGS was created in order to fill the gap that the founders felt had been left by GIA when it came to Cut.

If you’re looking for GIA and AGS Diamonds that have the best light performance and cut precision, take a look at the A CUT ABOVE Diamonds line from Whiteflash. If you’re looking for Fancy Shape Diamonds take a look at James Allen.

Diamond Color

When it comes to diamond color, GIA grades on a scale from D – Z, where D is colorless and Z refers to diamonds with a yellow or brown hue. For consistency, during the grading process, diamonds are viewed under strict lighting and compared to other diamonds with a known color.

Under the system, diamonds with a grading from D to R will feature slight variations in color that may not be visible to the naked eye. Diamonds with a grading of S to Z might feature color deformities that are visible to the naked eye.

AGS uses the same methodology for grading color and has established a number of practices to ensure it maintains consistency with GIA standards.

Unlike GIA, however, AGS uses a scale of 1 – 10, where 0 to 1.0 refer to a colorless diamond whilst, at the other end of the scale, 7.5 to 10 refer to a diamond with light coloring. There is a separate grading reserved solely for fancy yellow diamonds.

Take a look at my article on Diamond Color for more information.

Diamond Clarity

In terms of diamond clarity, GIA grades on a scale ranging from FL (flawless) where the diamond has no inclusions or visible blemishes under 10x magnification, through to I1, I2, and I3 (included), where the diamonds feature very obvious inclusions and are visible to the naked eye.

This scale of clarity has become recognized as an international standard, with many jewelers around the world using the GIA scale.

Again, AGS employs the same methodology, however, they grade on a scale of 0 to 10, with 0 aligning with GIA’s FL grading and 9.5 – 10 aligning with GIA’s I3 rating.

Head to my Diamond Clarity article for more information on the subject.

Diamond Cut

When it comes to Cut, GIA only offers an overall cut assessment on round brilliant diamonds and, to date, hasn’t announced any plans to extend their cut grading to any other shape.

AGS has developed the only scientifically verified system for analyzing the cut quality and light performance of diamonds which built upon their proportion based cut grade.

This scientific performance cut grade system is particularly useful as it can be applied across numerous diamond shapes and facet arrangements, including oval, princess, and cushion cuts, as well as the pioneering round brilliant cut. It considers more physical measurements of a diamond and uses more computer analysis that the previous  system.

Diamonds are graded on a scale of 1 through to 10 based on their cut, with 1 being the highest grading that can be given. Under this grading system, the consumer is given details of the diamond, including the cut, symmetry, shape, dimensions, carat, and profile, along with a final grading. As the AGS cut certification eliminates the need for human verification, it removes the subjectivity associated with color and clarity certification.

AGS’ cut certification is held in high regard, as this was the first lab to provide the Ideal Cut Grade for the Round Brilliant Diamond, also known as the AGS triple zero cut or AGS Ideal.

The labs’ individual approach to Cut grading can be traced back to their origins. Because GIA was founded by people working within the industry, they weren’t really interested in light performance as achieving the highest grading might involve sacrificing carat, which wasn’t in their interest. After all, no retailer would want to sacrifice the weight of a diamond to improve its cut as this would require more skill, time, and precision, making the process more expensive.

Although GIA have started offering a Cut grading, this is still extremely limited in comparison to AGS and so, for this reason, we’d only recommend using AGS for this aspect of the process. GIA should only be used if it comes complete with light performance images as their cut grading system is way to broad to get a comprehensive understanding of a diamond’s cut without it. Learn more about Diamond Cut here.

Diamond Carat

When it comes to assessing the Carat of the diamonds, both AGS and GIA follow a similar process. However, it’s important to note that whilst GIA only measures to 2 decimal points, AGS measures to three, making it slightly more precise.


There can be little doubt that both the AGS and the GIA are held in high regard when it comes to the grading certification of diamonds.

Rather than competing against each other to be the most respected or trustworthy source of certification, GIA and AGS should be viewed as sister organizations who, between them, provide a full and comprehensive certification process.

Whilst the two labs use different grading scales, which can seem confusing to inexperienced diamond buyers, measures are taken to try and ensure that the respective gradings are consistent. For example, both labs use master color sets for grading diamond color. The master sets used by AGS are graded by GIA and assembled according to the strict standards GIA has established for their use. In addition to this, AGS employees are actually usually GIA trained to ensure consistency across the board.

Although AGS is small in size in comparison to GIA, it is just as important, having established a name for itself as a pioneer in the critical area of diamond cut analysis, filling the gap left by the GIA. With this in mind, an AGS report provides more in-depth information relating to the cut quality and light performance of the diamond. As Cut is the most important of the 4Cs, it is perhaps not surprising that the lab’s attention to detail when it comes to this aspect of the certification process has fuelled the demand for services offered by AGS labs.

Taking all this into account, I would be happy to trust a diamond grading by either GIA or AGS, and would be confident in advising you to do the same. However, if you’re looking for more detailed information in terms of the cut of the diamond, AGS is likely to provide a more comprehensive and satisfactory certification service.  In fact, we would only ever recommend using a GIA cut grading if light performance images were also supplied as the lab’s cut grading is way too broad without them.

If you’re looking for GIA and AGS Diamonds that have the best light performance and cut precision, take a look at the A CUT ABOVE Diamonds line from Whiteflash. If you’re looking for Fancy Shape Diamonds take a look at James Allen or Blue Nile for great options.

Richard Jenkins, The Diamond Guru

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