In fact, AGS Laboratories was the first lab in the world to start issuing cut grades in their diamond grading certificates in 1996. The current AGS cut grading system was updated in 2005.
This means that if you purchase an AGS graded diamond, make sure the report date is after 1 June 2005. If you purchase a GIA graded diamond, make sure that it is after 1 January 2006. Interestingly, if you happen to have a GIA diamond that was graded prior to 2006, you can find your diamond’s cut grade under the new system using GIA’s report check tool. This is because although GIA didn’t put it on the report, they actually kept record of all this data internally.
GIA and AGS take very different approaches in their cut grading systems. GIA’s approach uses proportion metrics developed by researching over 70,000 diamonds to evaluate 7 cut parameters: brightness, fire, scintillation, polish, symmetry, durability and weight ratio. According to GIA, this approach focuses on the aspects of cut that make the round brilliant diamond look the way it does and the degree differences in proportions create observable distinctions.
AGS’s approach is based on light-performance. Light ray tracing software is used along with assessing computer generated ASET, fire, and scintillation maps over a range of tilt angles to determine the cut grade. The approach taken by AGS seems to be more focused on the theory of optics, relying more heavily on technology. This approach requires assumptions about the observer’s behaviour and tastes that may not fully represent reality.
In general, GIA seems to have a greater tolerance for steeper crown angles and bigger tables but tends to punish painted upper girdles more. Their view is that light leakage can be positive when it enhances contrast.
AGS takes the opposite view seeing certain kinds of painted upper girdles as a positive for light return.
Both labs publish cut tables that you can look up to see what crown and pavilion angles are predicted get you their top cut grade. These tables are meant as cutting guidelines so you can’t rely on them completely for grading but it should still give you a good idea.
Below is a summary of the data using the CA/PA combinations for a 56% table and a 61% table.
For GIA, the range of excellent CA/PA combinations for a 56% table ranges from 31.5/41.2 to 33.5/41.8 to 36.5/40.8. For AGS, the range of excellent CA/PA combinations for a 56% table ranges from 33.6/41.0 to 37.6/40.2.
For a 61% table, which is the upper-limit of the top grade for AGS, GIA goes from 32.5/41 to 35.5/40.8 and AGS goes from 32.2/41.2 to 33.0/41.0. The final thing to note is that GIA’s upper limit for table size is 62% whereas these diamonds will receive an AGS2 cut grade.
From this quick analysis, you can see that GIA allows a shallower crown angle and steeper pavilion angle when the table percentage is in the mid-range of ideal. GIA also allows steeper crown angles when the table percentage is on the upper-limit of the ideal range. On top of this, GIA has a greater tolerance for larger tables.
While GIA may be correct that these stones face up excellent, in most peoples opinion in the trade, there is a need to have a stricter cut grading system like AGS. There have been people in the trade who have done consumer tests on whether consumers prefer a slightly steep deep diamond versus one that has a painted girdle. Surprisingly, most consumers have been found to prefer the steeper and deeper diamond over the painted upper girdle, which agrees with GIA.
To clarify, both GIA and AGS agree that painting on the pavilion side leading to a dug out girdle is always detrimental to light performance and will not award an Excellent/AGS0 cut grade for these diamonds. The acceptable painting of the girdle by AGS is crown-only painting.
Should you pick AGS over GIA?
The short answer is no, and here is why. There are some practical benefits to purchasing a GIA graded diamond. First is that it is the most popular lab in the world, and increasingly diamonds are being sold based on the GIA certificate alone despite the fact that you know that the certificate is only half the story. This fact actually demonstrates why it is all the more important that you as a diamond prosumer understand the value of your diamond.
Also, the popularity of GIA diamonds mean that they are more liquid and this reflects positively on the resale value of the diamond and the ease of reselling that diamond. GIA also sets the standard for color, clarity, and fluorescence grading and their gemmologists are among the most trusted in the world.
What has been recommended is to take what you have learned from this and choose a GIA graded diamond that is also likely to have AGS Ideal light performance. By choosing GIA over AGS, you can have peace of mind regarding the color, clarity, and fluorescence grading of your diamond.
The good thing is that you already know how to do this. If you stick to the recommendations in the "picking a diamond" tutorial, then you should be well within the GIA Excellent and AGS0 grades.