GIA vs AGS
While GIA and AGS are considered two of the best diamond grading labs globally, due to recent inconsistencies in GIA's grading practices, we argue that the latter is better and more consistent in grading diamonds and offering accurate results.
While GIA is known to be a consistent lab in how they grade diamonds, our experience does not entirely support this assertion. Recently, we noticed inconsistencies and discrepancies in their grading practices. Unfortunately, as the lab has grown, it has struggled to maintain quality.
Bottom Line Recommendation:
- Both are reliable, but AGS is better.
- GIA is more consistent and stricter in grading the clarity of SI1/SI2 diamonds. AGS is improving its grading processes for lower clarity grades.
- GIA's cut grading process is inferior to AGS's proportion-based methodology. The latter's scientific light performance and proportion-based approach for assigning the cut grade are more accurate in assessing the cut quality. This can be remedied in GIA-graded diamonds if you know how to read their grading reports. Review our diamond proportions guide while reviewing a GIA-graded diamond.
- GIA's cut grade ranges from Excellent to Poor; the color range is D to Z; the clarity scale is flawless (FL) to include 3 (I3).
- AGS uses a 0–10 scale for cut, color, and clarity (0 being ideal and 10 being poor).
A diamond's certification or grading is one of the seven factors determining the price of a diamond. While the other six factors are explained below, this article explores the key differences between AGS (or AGSL) and GIA.
The Gemological Institute of America:
The Gemological Institute of America (GIA) was established in 1931 as a non-profit institute to protect buyers and sellers of gemstones by comprehensively researching and grading diamonds. It developed the 4Cs international grading system (Cut, Clarity, Carat, and Color) in 1953. It is headquartered in California and has 11 campuses, 9 laboratories, and 4 research centers across 13 countries.
GIA mainly offers two grading reports: diamond dossiers and full grading reports. Dossiers are small reports without a full clarity plot, often issued for small diamonds. On the other hand, the full report is a comprehensive document. As a result, it often provides sufficient information for professional dealers to accurately assess a diamond's quality based on the report without physically seeing it.
Given that GIA is mainly a non-profit educational institution, it is usually treated as a reliable lab; its graduate gemologists are also respected in the industry. Furthermore, its educational certificates are often benchmarks for how individuals professionalize/accredit their craft as graduate gemologists. Additionally, its 4Cs and continued research and publication continue to positively contribute to the flourishing and growth of the diamond industry.
GIA is still exploring a crucial subject before it decides to offer conclusive determinations/results: the cut grade. For example, it provides a cut grade in round-cut diamonds, but it has yet to assign it to other fancy cut stones, such as princess-cut or cushion-cut diamonds. In our opinion, GIA is still working out the best and final proportions and criteria before it gives these grades in its reports.
Even in round diamonds, GIA offers excellent cut grades in a wide range instead of AGS, which uses a specific proportion-based system. GIA argues that the proportions can be modeled differently to achieve optimal fire, brilliance, and scintillation. It also believes that a rigid proportions-based system is not the right approach to determine the cut grade of a diamond. Please read our detailed article on diamond proportions for further details.
As GIA has grown, its grading practices have become less consistent in recent times. For example, we recently submitted a big G VVS2 GIA graded diamond to be regarded after recutting it, and GIA graded it more as a lower H grade diamond (not honoring their own grading report).
The American Gem Society (AGS):
The American Gem Society (AGS) was established in 1934. It is a trade association of jewelers, traders, appraisers, and suppliers, operating primarily on a membership basis. As part of its commitment to learning and education, AGS created the American Gem Society Laboratories (AGSL) in 1996. AGS also has an advanced instruments division, founded in 2004.
AGS's diamond grading labs are only a few years old compared to GIA's, which have been around since 1931.
While AGS uses the same 4Cs model, it has developed grading criteria for diamonds. They also offer a variety of grading reports, with the light platinum grading report being one of the best.
AGS's proportions-based cut grading system is considered slightly superior to GIA since it uses precise measurements of cut, table, girdle, crown angle, pavilion angle, and cult to determine the cut of a polished diamond. However, at the same time, we should not entirely dismiss GIA's research-based position that proportions can be modeled differently based.
It is also helpful to share that AGS assigns a cut grade to fancy cut stones such as emerald-cut diamonds, but GIA does not offer such grades for fancy-shaped stones in their grading reports.
The clarity grade in AGS diamonds can be an issue in lower grade diamonds, such as SI1 and especially in the SI2 range. We have seen SI2-graded diamonds by AGS that GIA would not grade as SI2 but as I1 (included 1).
Conclusion on GIA vs. AGS:
While AGS and GIA are well-known diamond grading labs, it is fair to suggest that the former has the edge due to its improved and consistent grading system; the latter has shown inconsistencies. AGS's proportions-based cut system is better and more advanced in helping buyers select a well-cut diamond without having to do any extra work or research. However, this issue can be remedied if you know the right proportions for ideal brilliance and fire. Please follow our ideal proportions chart and buy a GIA-graded diamond that fits our suggested ideal or super-ideal proportions range.
The other six factors that affect the price of a diamond and how they are graded in GIA and AGS reports:
In addition to diamond grading reports, six other factors affect the price of a diamond, including shape, carat weight, cut, color, clarity, and fluorescence.
How does the shape of a diamond affect its price?
In short, shapes such as round or oval cuts are bigger in their overall diameter than square princess cuts in each carat weight range. For example, a well-cut 1 carat round diamond is usually 6.4-6.5mm in diameter, whereas a square princess cut 1 carat would be in the 5.3-5.5mm range. Therefore, it usually takes a larger rough unpolished diamond to get a 1-carat round polished diamond than a 1-carat princess cut diamond. Since a bigger rough diamond is needed to cut a 1-carat round diamond, the cost of obtaining it will add up to the overall price of the resulting diamond.
AGS and GIA reports are the same when it comes to the shape of the diamond. Nothing, in particular, to worry about.
How does carat weight affect the price of a diamond?
This is a no-brainer; the larger the stone, the higher the price. However, there is a catch to it. Since the price of natural diamonds is determined by how scarce the diamond is, the rarer the stone, the higher the price.
Given that larger diamonds are scarce, their prices jump significantly upwards as the carat weight increases. In other words, do not expect the price of a 1-carat diamond to be twice as much as the price of a 0.5-carat diamond (all other factors being equal), but 3 to 4 times higher. The same applies to 2, 3, or 4-carat diamonds. Larger gem-quality diamonds are not commonly found, so their prices significantly increase as the carat weight increases.
AGS and GIA reports are the same when it comes to the carat of the diamond.
What about the diamond cut?
The cut is the only C determined by human beings and not nature. The other 3Cs are naturally formed in a diamond.
The cut of a diamond is one of the most important factors to consider when shopping; poorly cut diamonds are too deep or shallow. Make sure you follow our instructions on the diamond cut while buying a diamond.
As explained earlier, AGS has a big advantage over GIA because of its proportion-based grading model.
How does color affect the diamond?
Diamond color is an essential factor in white diamonds. The higher the yellowish tint or hue in a stone, the less desirable it becomes. Diamond color ranges from D to Z, with the D-F color range considered colorless (the best) and G to J considered near colorless color grades. G is almost colorless—an ideal choice—and H and I are also excellent choices. Follow our diamond color chart for reference. Diamond fluorescence can be a helpful factor to consider while evaluating the color of a diamond. See why in the fluorescence section below.
GIA is stricter compared to AGS when grading color in white diamonds.
The importance of diamond clarity:
At a minimum, most people want their diamonds to be eye-clean. Diamond clarity assesses the pureness/cleanness of a diamond from natural inclusions. Since diamonds are formed under immense pressure beneath the earth's surface, most have inclusions in the form of crystals, clouds, feathers, and pinpoints. Hence, the fewer the inclusions, the better the diamond. GIA clarity scale grades diamonds between Flawless and Included 3 (I3). Flawless to VVS/VS1 are investment-grade diamonds, VS2-SI1 are excellent options for maximizing the size of a diamond without compromising on clarity, and SI2 could still be a budget option and, in some cases, also eye-clean. I1 to I3 are not considered eye-clean. Follow our diamond clarity scale for reference.
GIA grades clarity more strictly than AGS.
How to use diamond fluorescence to your advantage:
Fluorescence is generally a negative factor in a diamond and can affect its price, but you can use it to your advantage. It is a good idea, in general, to avoid fluorescence in D-G color diamonds (faint is okay). However, it can be a beneficial factor in H-above color grades because blue is a complementary color to yellow and can make a yellowish diamond look whiter than its actual color. The price of a diamond is discounted to a degree depending on the level of fluorescence it emits under ultraviolet rays. Fluorescence is graded as none, faint blue, medium blue, strong blue, and very strong blue. Rarely, fluorescence could also be in other colors, such as yellow or green, though it is best to avoid them. Please see our guide on diamond fluorescence for more details.
AGS and GIA are on the same page about diamond fluorescence, and there is nothing, in particular, to flag here.