“Are GIA Excellent and AGS Ideal cut comparable? How does GIA’s “symmetry” and “polish” come into play? Do I need all three of GIA’s symmetry, polish and cut grades to be GIA Excellent for the diamond to be comparable in light performance to AGS Ideal cut diamonds? Thank you.”
People tend to assume that GIA Excellent cut diamonds and AGS Ideal-0 cut diamonds are comparable. However, that is not really the case as you will soon discover. But not to worry, by the time you finish reading this article, you will know how to select the best GIA Excellent and AGS Ideal cut diamonds with supreme confidence!
Is GIA Excellent Comparable to AGS Ideal?
Both the Gemological Institute of America (GIA) Gem Trade Laboratory (GIA-GTL) and the American Gem Society Laboratory (AGSL) grade comparably for diamond polish and symmetry, thus in my experience a diamond with a polish and symmetry grade of GIA Excellent is likely to receive a polish and symmetry grade of AGS Ideal-0 from the AGS Laboratory.
- GIA Excellent Polish vs AGS Ideal Polish = Comparable.
- GIA Excellent Symmetry vs AGS Ideal Symmetry = Comparable.
- GIA Excellent Proportions vs AGS Ideal Proportions = NOT Comparable.
The GIA and AGS gemological laboratories grade diamond proportions differently, and I find that the range of proportions relied upon by the GIA to be much broader and less reliable than those employed by the AGSL. However it should be noted that I also feel that the range of proportions relied upon by the AGS Laboratory for their zero ideal cut proportions rating to also be a bit broad, and thus I recommend my clients adhere to the following “best range of proportions for an ideal cut diamond” when selecting a round brilliant cut diamond:
- Total Depth between 59 – 61.8%
- Table Diameter between 53 – 58%
- Crown angle between 34.3 – 34.9 degrees*
- Pavilion angle between 40.6 – 40.9 degrees
- Lower Girdle Facet (LGF) between 75 – 78%**
- Girdle thickness between 0.7% thin to slightly thick
- Culet size: GIA “none” or AGS “pointed” (same thing)
* Effect of Crown Angle on Diamond Sparkle:
The crown angle of the diamond determines the balance of brilliance (white sparkle) and dispersion (colored sparkle) and thus it is largely a matter of personal preference. I’ve been known to recommend diamonds with a crown angle as high as 35.0 degrees, and as low as 34.0 degrees, if it is offset by the right pavilion angle. I recommend consulting with me on a stone-by-stone basis because a diamond is essentially a puzzle constructed on a sliding scale of sorts, which changes accordingly as each piece of the puzzle is cut / polished; and since my services are free for consumers, there is no reason not to.
** How Lower Girdle Facet Length Affects Diamond Sparkle:
Diamonds cut to the proportions outlined above, that feature Lower Girdle Facets (LGF) in the range of 75 – 78% tend to exhibit “broad spectrum sparkle” which means that it is larger in size, and bolder, brighter, and more vivid, than a diamond of the same proportions would exhibit if the LGF’s were in the range of 80 – 82% which tends to produce “pin-fire sparkle” that is smaller in size.
While this is largely a matter of personal preference, our human eyes tend to have difficulty dispersing smaller flashes of white light into colored light/fire, because of the way that our eyes clip the light; thus round brilliant cut diamonds that exhibit pin-fire type sparkle, tend to appear to be more brilliant, by exhibiting a higher concentration of white sparkle than exhibit a virtual balance of brilliance and dispersion.
A good example of diamonds that are cut to produce broad-spectrum sparkle are Brian Gavin Signature Hearts and Arrows round brilliant cut diamonds, and the Crafted by Infinity Hearts and Arrows round brilliant cut diamonds which are marketed online by High Performance Diamonds.
How Important is the Polish grade of a diamond:
Diamond polish grades are a reflection of how fine the degree of polish that has been applied to the surface of the diamond, it also tends to be an indication of the skill level of the diamond cutter who cut the diamond, and I definitely prefer that diamonds have a polish grade of either GIA Excellent or AGS Ideal.
The diamond polish will affect the luster of a diamond. The difference between Excellent / Ideal / Very Good polish can be difficult to detect without magnification. However, it is important in terms of transparency and affects how light bounces off of the surface of the diamond.
I suppose that the easiest way to determine whether you should purchase a diamond with AGS Ideal, GIA Excellent or Very Good Polish, is to ask whether you would purchase a new car with scratches in the paint?
A very good paint job might look fine when you look at the diamond from a few feet away, but an excellent paint job like the one on this Bentley coupe will look exceptional when inspected closely. If you’ve ever been to a car show and had the opportunity to see those cars with exceptional paint jobs, you know the difference that it has upon the overall look of the car.
And of course there is the peace of mind that comes with knowing that the diamond you purchased has the very best polish possible… the only time I would consider buying a diamond with very good polish, would be in the instance of a fancy shape diamond, and only if everything else was spectacular in terms of the proportions and symmetry grades, because it can be extremely difficult to find fancy cut diamonds with both Excellent / Ideal polish and symmetry.
How important is the Symmetry grade of a diamond:
Diamond symmetry grades are based upon “meet point symmetry” which is based upon the degree of precision that the facet junctions meet up with one another at the points, this is not the same as “optical precision” (sometimes referred to as optical symmetry) which refers to the degree of precision that the facets were polished on to the surface of the diamond, which happens to be something that neither the GIA or AGSL grade at the moment.
If you really want to ensure that the diamond you purchase is going to deliver the highest volume of Light Performance and Sparkle Factor, then it is critical that the diamond exhibit a crisp and complete pattern of hearts and arrows, like the pattern exhibited by this 1.687 carat, I-color, SI-1 clarity, Brian Gavin Signature round diamond. It is equally important to obtain images of any diamond you are considering, as seen through an ASET Scope, which provides insight into how bright the diamond will be, as well as an indication of how evenly light is being distributed throughout the diamond; and an Ideal Scope, which is designed to provide an indication as to the degree that the diamond is leaking light.
Both Brian Gavin and High Performance Diamonds provide for every diamond in their exclusive in-house inventory of super ideal cut diamonds, they do so in order to demonstrate that their diamonds are the very best. The production of a crisp and complete pattern of hearts and arrows like the one pictured above requires that the facets of the diamond be truly symmetrical, evenly sized, and evenly spaced/indexed around the diamond from the perspective of three hundred and sixty degrees!
Even the slightest variance in the length, size, or alignment of the facets will throw off the pattern… Take this next diamond for instance:
Enchanted Diamonds Reviews: GIA 2176526472
This 1.61 carat, I-color, SI-1 clarity, round from
Enchanted Diamonds has an overall cut grade of GIA Excellent, which means that the polish, symmetry, and proportions grades are all GIA Excellent. The diamond has a pavilion angle of 40.8 degrees, which is properly combined with a pavilion depth of 43% and this means that the diamond will exhibit a high volume of light return. The 35.0 degree crown angle is a good offset for this pavilion depth and will produce a pretty good balance of brilliance (white sparkle) and dispersion (colored sparkle).
Effects of Azimuth Shift and Facet Yaw:
The 75% lower girdle facet length on this 1.61 carat, I-color, SI-1 clarity, round from
Enchanted Diamonds (bankrupt 2019) should produce the Broad Spectrum Sparkle that I look for in a round brilliant ideal cut diamond. However, it’s not going to. Take a look at how fat the pavilion main facets appear in the clarity photograph. What’s up with that? And why are three of the pavilion main facets (arrow shafts) translucent instead of black like the others? And if this diamond has a symmetry grade of GIA Excellent, why do the arrowheads seem to be offset from the arrow shafts? The short answer is that this GIA Excellent cut diamond exhibits signs of Azimuth Shift and lousy optical precision!
Azimuth Shift is the term used by professional diamond graders to refer to adjustments that a diamond cutter made to the indexing of the facets polished on to the surface of a diamond, which creates a variance in the size of the facets and alters the pattern of light reflection throughout the diamond.
The term “Facet YAW” was coined by fifth-generation diamond cutter Brian Gavin, to refer to the effect created on the surface of a diamond, when a facet is polished in the wrong direction, which creates a distortion in the angle of the facet because one side of the facet was in contact with the diamond cutting wheel more than the other edge of the facet… This concept will be explained more in-depth in the diamond grading tutorial on Azimuth Shift and Facet YAW.
This 1.61 carat, I-color, SI-1 clarity, round from
Enchanted Diamonds might have an overall cut grade of GIA Excellent, and it looks pretty darn good “by the numbers” but it exhibits all the telltale signs of Azimuth Shift and Facet Yaw in the clarity photograph and reflector scope images, which is why it is so important to obtain photographs of the diamond as seen through an ASET Scope, Ideal Scope, and Hearts & Arrows scope if light performance and sparkle factor is an important feature to you. Notice how erratic the distribution of red, green, and blue are throughout the ASET Scope image for this diamond, the telltale signs of Azimuth Shift can not be ignored. GIA Excellent Cut, yea…
And things don’t look any better in the Ideal Scope image provided for this 1.61 carat, I-color, SI-1 clarity, round from
Enchanted Diamonds either… I’m curious whether you happened to notice how erratic the spacing is between the arrows? Wider in some places, and narrower in others, it is particularly noticeable between the arrows located in the relative four o’clock and six o’clock positions. Despite the ideal proportions and overall cut grade of GIA Excellent, there is no way that this is going to be a top-performing diamond! If you were to think of the upper and lower halves of this diamond are like a jar, the lid is basically screwed on wrong! If this were a car, it might be recalled under the Lemon Law!
Obviously I’m not impressed with the degree of optical precision exhibited by this 1.61 carat, I-color, SI-1 clarity, round from
Enchanted Diamonds; but come on, just look at how completely whacked the arrows pattern looks in this reflector scope image! But truth be told, I have to hand it to Enchanted Diamonds for trying to provide reflector scope images for some of the diamonds in their virtual inventory because without this sort of information, we wouldn’t have been able to determine that this GIA Excellent cut diamond has lousy optical precision. We might have wrongly assumed that it would be a top-performing diamond simply because it has an overall cut grade of GIA Excellent.
Last but not least, let’s take a look at the hearts pattern exhibited by this 1.61 carat, I-color, SI-1 clarity, round from
Enchanted Diamonds, which has an overall cut grade of GIA Excellent. Notice that Enchanted Diamonds does not dare refer to this as a “Hearts and Arrows Diamond” but rather refers to the image under the heading “Hearts View” because it was taken through a Hearts and Arrows Scope, but clearly the pattern looks like a bunch of misshapen lawn darts more than hearts, they are deformed, malformed, distorted, crooked, twisted, warped, out of shape, bent, asymmetrical, irregular, mis-proportioned, ill-proportioned and clearly disfigured. GIA Excellent huh?
Well, the good news is that not all GIA Excellent cut diamonds exhibit optical precision as bad as this 1.61 carat, I-color, SI-1 clarity, round from
Enchanted Diamonds. In fact, I often find some very nice looking GIA Excellent cut diamonds when I search Enchanted Diamonds inventory but have yet to find anything that compares to the degree of optical precision exhibited by Brian Gavin Signature round diamonds, or those produced by Crafted by Infinity.
The reason for the difference in overall cut quality between
Enchanted Diamonds, Brian Gavin, and Crafted by Infinity is because Enchanted Diamonds is essentially a clearinghouse, which operates by offering a virtual inventory of diamonds that is based upon the global inventory provided by the various multiple listing services that we use as an industry to market diamonds worldwide; whereas both Brian Gavin and Crafted by Infinity are diamond cutters, who both specialize in producing diamonds of the highest cut quality.
Brian Gavin Signature diamond reviews: AGS 104074030045
With that in mind, let’s take a more in-depth look at this 1.687 carat, I-color, SI-1 clarity, Brian Gavin Signature Hearts and Arrows round brilliant cut diamond, which has an overall cut grade of AGS Ideal-0 as determined on the Light Performance grading platform of the AGS Laboratory, which uses Angular Spectrum Evaluation Technology (ASET) to measure diamonds for brightness, and which provides us with an indication of how evenly light is being distributed throughout this Brian Gavin Signature diamond. Notice how the arrows pattern is evenly spaced and exhibits a symmetrical pattern; and how consistent the distribution of red, green, and blue is throughout the diamond. “By the numbers” this puppy should have a sparkling personality, with a high volume of light return, a virtual balance of brilliance and dispersion, and broad-spectrum sparkle! Sunglasses required!
Notice how much more crisp and vibrant the Ideal Scope image for this 1.687 carat, I-color, SI-1 clarity, Brian Gavin Signature round diamond is, as compared to the Ideal Scope image provided for the 1.61 carat, GIA Excellent cut diamond reviewed previously. This diamond exhibits the highest degree of optical precision, and I have no doubt that it will be drop-dead gorgeous! And it’s worth mentioning that there are a lot of AGS Ideal-0 cut diamonds that fail to meet my selection criteria for proportions and optical precision, thus it isn’t so much a matter of whether the diamond has an overall cut grade of AGS Ideal or GIA Excellent, as much as the combination of proportions and optical precision.
AGS Ideal-0 Cut Diamonds vs GIA Excellent Cut Diamonds:
Hopefully, this diamond buying tutorial on the differences between AGS Ideal-0 Cut Diamonds vs GIA Excellent Cut Diamonds has helped you to develop a better understanding on the differences and similarities of diamonds graded by the AGS Laboratory and the GIA Gem Trade Laboratory.
Regardless of whether a diamond is GIA Excellent cut or AGS Ideal cut, you now understand the important insight provided by reflector scope images, such as ASET Scope, Ideal Scope, and Hearts & Arrows scope images; and know what to look for in terms of Optical Precision and Azimuth Shift.
But know that you don’t have to try and interpret all of this diamond grading data on your own. In fact, you don’t have to search for diamonds at all if you don’t want to, you can take advantage of my free Diamond Concierge Service and leave the diamond shopping to me!
I’ve got almost 30 years of experience buying diamonds professionally… egads, it will be 30 years this October! And your price on the diamonds that I recommend is not affected by using the link encoded with my Affiliate ID to order the diamond, because my fees are paid out of the annual advertising budget of each vendor. How cool is that? All you need to do is drop me a note indicating the range of carat weight, color, clarity, fluorescence, and price that you are willing to consider, and I’ll take care of the rest!
Which online diamond vendors provide the best detail:
Brian Gavin Diamonds provides one of the most comprehensive and in-depth diamond details pages that I’ve ever seen, complete with a high-resolution video of the diamond, which can be enlarged by clicking on the four white arrows that appear in the upper right-hand corner of the video window. Plus a high-resolution clarity photograph of the diamond, ASET Scope, Ideal Scope, and Hearts & Arrows scope images where applicable, so that you can verify the exceptional optical precision.
A copy of the diamond grading report is also provided, but I prefer to click on the link to “Verify” the diamond grading report that is provided in the right hand column of the diamond details page, which will open up a new tab in your browser, and load the details for the diamond grading report as provided by the American Gem Society; then click on the link at the bottom of that page to open the diamond grading report, and you will be presented with a full size copy of the Diamond Quality Document, which can be resized so that you will be able to identify the inclusions within the diamond quickly and easily.
Crafted by Infinity diamond reviews: AGS 104075137004
High Performance Diamonds which is the online distributor for the hearts and arrows diamonds produced by Crafted by Infinity, also offers an in-depth evaluation of the diamonds that they offer, complete with a high-resolution clarity photograph, and images of the diamond as seen through an ASET Scope, Ideal Scope, and a Hearts & Arrows scope. This is the diamond details page for a 1.606 carat, I-color, VS-2 clarity, Crafted by Infinity diamond that will undoubtedly knock your socks off! It has an overall cut grade of AGS Ideal-0 and looks fantastic in all of the scope images.
This diamond exhibits a crisp and complete pattern of hearts and arrows, which is proof that the diamond exhibits superior optical precision; and the 40.8 degree pavilion angle is going to provide a high volume of light return, while the 34.3 degree crown angle provides a virtual balance of brilliance and dispersion; and the 77% lower girdle facet length ensures that the sparkle will be broad-spectrum! Which means that the sparkle is going to be larger in size, bolder, brighter, and more vivid, than what a lot of ideal cut diamonds exhibit…
While High Performance Diamonds does not routinely provide video of the diamonds on their diamond details page, Wink will provide video upon request, so all you have to do is ask.
James Allen Diamonds provides limited details:
Unless you happen to be considering one of their True Hearts Diamonds, James Allen only provides a high-resolution video for the ideal cut diamonds in their virtual inventory, the diamond details page provided for this 1.61 carat, I-color, SI-1 clarity, GIA Excellent round diamond from James Allen is pretty bare-bones, but the clarity video is sufficient enough for me to be able to identify the needle-shaped diamond inclusions in the nine o’clock region.
And I am able to barely detect some of the other inclusions located in that region if I use my mouse to drag the diamond back and forth, everything appears to be translucent, and that is wonderful because the odds are that this SI-1 clarity diamond is going to be eye-clean; or at least the inclusions are going to be pretty difficult to locate with just my eyes since they are translucent in tone/color.
By the numbers the diamond should be quite pretty, the 40.8 degree pavilion angle should produce a high volume of light return, while the 35.0 degree crown angle is likely to produce a pretty good balance of brilliance and dispersion, and the 75% lower girdle facet length should provide broad-spectrum sparkle. However we can’t really tell anything about the optical precision of the diamond without reflector scope images, and they are not routinely provided for diamonds which are not part of the James Allen True Hearts collection, even then you will be provided with only a small image of the diamond as seen through a hearts scope, and an ideal scope.
There have been times in the past when James Allen has provided my clients with an Ideal Scope image for a diamond like this 1.61 carat, I-color, SI-1 clarity, GIA Excellent cut diamond from James Allen, but they recently told one of my clients that they don’t provide ASET images.
Enchanted Diamonds, the mixed bag of diamond details:
Enchanted Diamonds is working off of virtual inventory, they are only able to provide whatever images each supplier is capable of providing them with, which will be nothing in some cases, and quite complete in other instances, thus it is kind of a crapshoot as to what you can expect from Enchanted Diamonds in terms of diamond details.
For instance, while the diamond details page for the 1.61 carat, I-color, SI-1 clarity, GIA Excellent cut round from
Enchanted Diamonds reviewed above was pretty thorough, the diamond details page for this 1.75 carat, J-color, VS-2 clarity, GIA 3X cut round from Enchanted Diamonds offers nothing but a copy of the diamond grading report, and you have to register with Enchanted Diamonds in order to view the lab report or images.
By the numbers, this GIA Excellent cut diamond should provide a high volume of light return, because of the 40.6 degree pavilion angle, however it should be noted that the pavilion depth of 43% would be better suited for a pavilion angle of around 40.8 – 40.9 degrees, and a pavilion depth of 42.5% would be better suited for the pavilion angle of 40.6 degrees; so I would definitely want to see an ASET Scope and Ideal Scope image before ordering this diamond.
Now there have been times in the past where I’ve sent a message via Skype to Jonathan and Joshua, inquiring whether they could obtain images on a diamond like this 1.75 carat, J-color, VS-2 clarity, GIA 3X cut round from
Enchanted Diamonds and received them within minutes, the images were already on file, but they simply hadn’t had time to update the diamond details page, and there have been other times when the images were simply not available, so it’s worth taking the time to ask.
Getting back to the proportions of the diamond, so I was saying that the pavilion angle of 40.6 degrees should provide a high volume of light return, but that I’d prefer the pavilion depth of the diamond be closer to 42.5%. The crown angle of 35.0 degrees should provide a pretty good balance of brilliance and dispersion, but the crown height of 15.5% is a little bit steep for that crown angle, and the 80% lower girdle facet length is going to produce that pin-fire type sparkle that is smaller in size, and which our eyes tend to have difficulty dispersing into colored light.
Thus this 1.75 carat, J-color, SI-1 clarity, GIA 3X round diamond from Enchanted Diamonds might be a good option for people who prefer that disco ball type of sparkle, but we have no way to judge the extent of the inclusions, nor the degree of optical precision; so we’re going to want to see reflector scope images of the diamond before making a decision, to ensure that this GIA Excellent cut diamond exhibits better optical precision than the 1.61 carat, I-color, SI-1 clarity, GIA 3X round from Enchanted Diamonds reviewed above.
Unfortunately images are not available for this diamond, I checked with Enchanted Diamonds and they verified that images can not be obtained; so you’ll be buying this diamond based upon the numbers alone if it is of interest to you; and this is exactly what I’m referring to when I say that the diamond details pages and the degree of diamond details that you can expect from Enchanted Diamonds is a mixed bag.
Ritani provides next to nothing in terms of diamond details:
A quick search for diamonds on Ritani will reveal that they provide clarity images and video for some of the diamond in their physical inventory. However, a stock clarity photograph is used most of the time, so you will have to request photographs and wait while they are provided. I’ve had limited success obtaining reflector scope images, but it’s pretty hit and miss. Notice how the diamond details page for this 1.61 carat, I-color, VS-2 clarity, GIA Excellent cut diamond from Ritani features a stock image of a diamond, and a link to the diamond grading report, that’s it folks!
Now in this particular instance, the details provided by the diamond grading report are enough to tell me that there is no need to obtain a clarity photograph, or reflector scope images for this 1.61 carat, I-color, VS-2 clarity, GIA 3X cut diamond from Ritani, because the proportions do not meet my selection criteria.
The 41.0 degree pavilion angle is combined with a pavilion depth of 43.5% which happens to be the critical tipping point where light begins not to strike fully off of the pavilion facets, thus this diamond is not going to exhibit the highest volume of light return; especially when you realize that those measurements represent the average of eight measurements, thus the pavilion depth is actually going to be higher than 43.5% at some point.
And the crown angle of 34.0 degrees is on the shallow side of my preferred range of 34.3 – 34.9 degrees, and it is combined with a crown height of 15.5% which would be better suited for a crown angle steeper than 35.0 degrees, and so even though the diamond has 75% lower girdle facets, it’s not going to deliver the type of sparkle that I’m looking for in an ideal cut diamond.
This is not to say that Ritani does not occasionally have ideal cut diamonds that meet my selection criteria, only that this one does not, and it was the only option available in this range of diamond characteristics when I ran a search for diamonds on Ritani this morning using my normal parameters.
Blue Nile provides limited details for Astor:
Blue Nile provides a clarity photograph, and some scope images for their Astor by Blue Nile, but does not provide any sort of images for other diamonds represented in their virtual inventory. I have been able to obtain images from the MLS data for some of the diamonds found in their inventory, so it’s worth asking me; just send me a link to the stone via email diamonds[at]niceice.com with a subject heading like “The Your Last Name Diamond Project” or the Blue Nile inventory number which will look something like LD05623868, which is the stock number for this 1.61 carat, I-color, SI-1 clarity, Blue Nile Signature round diamond.
Blue Nile Signature round diamond reviews: GIA 7198080165
I suppose that since I tripped across this 1.61 carat, I-color, SI-1 clarity, Blue Nile Signature round diamond while looking for an example of what their diamond inventory numbers look like, that I might as well take a moment to review it since it’s another example of a GIA Excellent cut diamond. The 41.0 degree pavilion angle is one-tenth of a degree beyond my preferred range, and yes it will have an effect upon the light return of the diamond, making it a bit less than the option presented by Brian Gavin even though it has a 43% pavilion depth.
The 34.5 degree crown angle is fantastic, and it normally would provide a virtual balance of brilliance (white sparkle) and dispersion (colored sparkle) however the 80% lower girdle facet length is going to produce pin-fire type sparkle which is smaller in size; and as stated previously, our human eyes tend to experience difficulty dispersing smaller flashes of white light into colored light/fire, and thus this diamond is apt to appear to be more brilliant than exhibit a virtual balance of brilliance and dispersion… However, that’s not exactly a bad thing, because lots of people prefer the icy bright white look that this combination of proportions provides.
The GCAL report issued for this 1.61 carat, I-color, SI-1 clarity, Blue Nile Signature round diamond provides you with a clarity photograph of the diamond, and images of the diamond as seen through a hearts and arrows scope, and what GCAL calls an optical symmetry analysis. There is some variance in the size and shape of the hearts, and they bleed into the arrowheads in some places, and bend a bit in the tips, but this is significantly better than the GIA Excellent cut from Enchanted Diamonds reviewed previously.
There are also minor signs of Azimuth Shift in the arrows pattern exhibited in the “Optical Symmetry” image provided on the GCAL report, and which indicates that the optical symmetry of the diamond is GCAL Excellent, but once again this is a matter of semantics and there is a difference between meet point symmetry and what I refer to as Optical Precision… All in all this 1.61 carat, I-color, SI-1 clarity, Blue Nile Signature round diamond is not a bad option as far as GIA Excellent cut diamonds go, but personally I’m leaning towards the 1.687 carat, I-color, SI-1 clarity, Brian Gavin Signature round diamond, or the 1.606 carat, I-color, VS-2 clarity, Crafted by Infinity diamond that because I’m a total cut nut, and either one of these would make me happy!