GIA vs. AGS cut grading?

[soxfan]

I couldn't wait to get my diamond home and plug in the numbers.... It is a GIA graded "very good cut." When I put in the numbers into AGS it came back as "ideal." I was under the impression that GIA was the "harder" grading system? But I've been combing the internet and have seen that most are saying it's the opposite in regards to cut. Is this accurate?

 

[JulieN]

Post the report number and carat weight.

 

[greenpang]

What are the numbers?

 

[soxfan]

here is the link...

http://www.octonus.com/oct/mss/gia-agspgs.phtml

so the table is 61%
pavilion angle is 41.8%
crown angle is 30.5%

I *think* I did it right?

 

[arkieb1]

Do you think we can hug and make up so I can answer your questions? I had the flu the other day and what I said was out of line. Sorry. I asked almost immediately for the moderators to remove my comments but they didn't.

Anyway - have you tried to tools bar up on the right here? Plug the numbers on the certificate into the HCA and the AGA/NAJA tools, that will give you some more information.

I think GIA is definitely stricter colourwise, I thought they are similar cutwise, both are helpful.

 

[SB621]

What is the depth?

 

[soxfan]

Do you think we can hug and make up so I can answer your questions? I had the flu the other day and what I said was out of line. Sorry. I asked almost immediately for the moderators to remove my comments but they didn't.

Anyway - have you tried to tools bar up on the right here? Plug the numbers on the certificate into the HCA and the AGA/NAJA tools, that will give you some more information.

I think GIA is definitely stricter colourwise, I thought they are similar cutwise, both are helpful.

Sure. I was expecting to hear "what the hell does it matter? You already bought the diamond!" Which I did. And I LOVE It!!!!!!

Here are the numbers:

GIA cert # 5131937087
round brilliant
8.23x8.36x4.96
2.04 ct
color K
Clarity SI2
cut: very good
total depth: 59.8%
table size: 61%
crown angle: 30.5%
crown height: 11.5%
pavilion angle: 41.8
pavillion depth: 44.5%
girdle: medium to thick, faceted (3.5%)
culet: none
polish: good
symmetry: very good
flor: none

 

[soxfan]

the HCA was 3.4 "very good" for the price....

 

[SB621]

I was expecting to hear "what the hell does it matter? You already bought the diamond!" Which I did. And I LOVE It!!!!!!

That would have actually been my response to you.

HCA put it at a 3.4

I think the general consensus is that AGS ideal cut grade has stricter requirements then GIA excellent. From what I have read GIA grades their cut on proportions/ measurements. AGS does on proportions and performance. Since your HCA score is over 2 (which isn't always a bad thing) I can see this not getting ideal or excellent cut.

Hope that makes sense. You can do a general search on PS and you will find many posts that discuss this topic and goes into more detail. If you search for posts by Stone Cold, who used to be very active on PS a few years ago, I know he did very detailed posts on MRB's on AGS vs. GIA cut.

EDIT: for grammar and spelling...sorry shouldn't type when I'm exhausted

 

[JulieN]

Very simple answer to your question: there are fewer combos that make the AGS top grade than GIA's top grade. There are some combos that are in AGS top grade that does not qualify for GIA EX.

 

[soxfan]

got it. I guess I don't have a very discerning eye. I guess the numbers don't matter THAT much if I love the stone. I just wondered about the grading because there are SOOOO many different sites that say so many different things in regards to the grading....

 

[MaximusCruiser]

Hmm... not quite sure why you said the diamond came back under AGS Ideal... I would have thought that the large table size of 61% will have ruled it out under AGS ideal since to meet that range, it must be no more than 57.5%. Am I missing something?

 

[soxfan]

Hmm... not quite sure why you said the diamond came back under AGS Ideal... I would have thought that the large table size of 61% will have ruled it out under AGS ideal since to meet that range, it must be no more than 57.5%. Am I missing something?

nope. See for yourself...

http://www.octonus.com/oct/mss/gia-agspgs.phtml

 

[soxfan]

here's the charts. You have to scroll down a bit. 61% table/41.8% pavilion angle/30.5% crown angle

GIA very good
AGS ideal

 

[delight]

I won't even consider a stone with a good polish. Let alone such angles.

 

[soxfan]

I won't even consider a stone with a good polish. Let alone such angles.

I searched last night and the consensus is that you cannot distinguish between excellent, very good or good polish with the naked eye. But anyway, I asked about GIA vs. AGS cut grading, that was all. I am happy with the diamond. I didn't ask you what stones you "won't consider."

 

[diamondseeker2006]

AGS has a far narrower ideal cut grade than GIA Excellent. A fraction of GIA Excellent stones will fit into AGS Ideal. That is why we use the HCA to narrow down the better GIA excellent stones.

Were you able to get an ideal scope image of this stone?

If you look at it from the side, it should have a pretty low and smaller crown due to the low crown and large table. It is partly for that reason that most people who like ideal cut stones would not buy it. That's just going to be the way it is on a board that specializes in ideal cut stones. But certainly, if you love the stone, that is all that matters!

 

[soxfan]

AGS has a far narrower ideal cut grade than GIA Excellent. A fraction of GIA Excellent stones will fit into AGS Ideal. That is why we use the HCA to narrow down the better GIA excellent stones.

Were you able to get an ideal scope image of this stone?

If you look at it from the side, it should have a pretty low and smaller crown due to the low crown and large table. It is partly for that reason that most people who like ideal cut stones would not buy it. That's just going to be the way it is on a board that specializes in ideal cut stones. But certainly, if you love the stone, that is all that matters!

Thanks diamondseeker. I guess the thing that threw me off was the VG on one and ideal on the other. Something didn't add up. I don't have an ideal scope, and the diamond is heading to VC today, so there's really no use scoping it. It's from our local jeweler. I don't think any of the jewelers around have them? I had never heard of ideal scope until I came here...

so are you saying that there are GIA "excellent" graded diamonds that people would turn down because of an HCA score? And how could a diamond that falls under AGS ideal score higher than a 2 on the HCA. Sorry, I am not the most diamond educated person. I spent most of my time researching my setting.

And lastly, did I READ the chart correctly? Do I actually have a GIA VG AND an AGS Ideal?

 

[SB621]

AGS has a far narrower ideal cut grade than GIA Excellent. A fraction of GIA Excellent stones will fit into AGS Ideal. That is why we use the HCA to narrow down the better GIA excellent stones.

Were you able to get an ideal scope image of this stone?

If you look at it from the side, it should have a pretty low and smaller crown due to the low crown and large table. It is partly for that reason that most people who like ideal cut stones would not buy it. That's just going to be the way it is on a board that specializes in ideal cut stones. But certainly, if you love the stone, that is all that matters!

Thanks diamondseeker. I guess the thing that threw me off was the VG on one and ideal on the other. Something didn't add up. I don't have an ideal scope, and the diamond is heading to VC today, so there's really no use scoping it. It's from our local jeweler. I don't think any of the jewelers around have them? I had never heard of ideal scope until I came here...

so are you saying that there are GIA "excellent" graded diamonds that people would turn down because of an HCA score? And how could a diamond that falls under AGS ideal score higher than a 2 on the HCA. Sorry, I am not the most diamond educated person. I spent most of my time researching my setting.

And lastly, did I READ the chart correctly? Do I actually have a GIA VG AND an AGS Ideal?

nevermind**

 

[Christina...]

AGSL determines cut based kn proportions and performance. Its possible to have a stone that meets AGSLs proportion guidelines but not pass its performance testing. In order to receive its ideal cut grade the stone must also have ideal polish and symetery. The only people who can determine if your stone is an AGS ideal stone is AGS.

 

[soxfan]

Hope that makes sense. You can do a general search on PS and you will find many posts that discuss this topic and goes into more detail. If you search for posts by Stone Cold, who used to be very active on PS a few years ago, I know he did very detailed posts on MRB's on AGS vs. GIA cut.

EDIT: for grammar and spelling...sorry shouldn't type when I'm exhausted

THIS has been very helpful! I am reading through the posts now. One of them explains why a higher HCA score diamond may look more attractive to some....Thanks SB!!!

 

[soxfan]

AGSL determines cut based kn proportions and performance. Its possible to have a stone that meets AGSLs proportion guidelines but not pass its performance testing. In order to receive its ideal cut grade the stone must also have ideal polish and symetery. The only people who can determine if your stone is an AGS ideal stone is AGS.

Gotcha. So the chart means nothing without AGS actually SEEING the diamond....

 

[SB621]

AGSL determines cut based kn proportions and performance. Its possible to have a stone that meets AGSLs proportion guidelines but not pass its performance testing. In order to receive its ideal cut grade the stone must also have ideal polish and symetery. The only people who can determine if your stone is an AGS ideal stone is AGS.

Gotcha. So the chart means nothing without AGS actually SEEING the diamond....

Correct.

 

[Christina...]

AGSL determines cut based kn proportions and performance. Its possible to have a stone that meets AGSLs proportion guidelines but not pass its performance testing. In order to receive its ideal cut grade the stone must also have ideal polish and symetery. The only people who can determine if your stone is an AGS ideal stone is AGS.

Gotcha. So the chart means nothing without AGS actually SEEING the diamond....

Right! The chart is simply a guide. I think that Ive actually read where a stones proportions fell outside of this guideline and AGS still gave the stone an ideal cut grade based on its performance and all around beauty, but I can't recall where I may have read that.

 

[soxfan]

Thanks everyone! Now I totally understand that I have an all-around Very Good diamond! On my way to the post office to mail this sucker to be halo'ed...

 

[DelsFan]

Thanks everyone! Now I totally understand that I have an all-around Very Good diamond! On my way to the post office to mail this sucker to be halo'ed...

Sounds like you got a very nice diamond. (Some) people here are passionate, particular, and (who, me?) obsessive. As with any purchase, cars, men's suits, women's gowns, even wedding china - for two or three times the cost you probably could have gotten a very slightly better product. "It's" got to stop somewhere, and it sounds like you did a good job without breaking the bank!

Post photos of your ring when it is complete; I'm sure everyone here would like to see it.

PS: And, by the way, while an HCA of 2 or better is ideal (pun intended), I doubt most of the best diamonds you could see in a store would touch 4, and many I've seen in malls would, seriously, it looks like they would have HCA's in the double digits.

 

[John P]

I am late here: Some days ago I was asked via Facebook to comment on this thread. While delayed, I can provide a few notes.

First: The conclusions of the original poster are accurate, relative to the world of cut-quality. The following comments are simply clarifications for future readers.

1. RE Girdle & Finish: << GIA cert # 5131937087 ... girdle: medium to thick, faceted (3.5%) ... symmetry: very good >>

Please note that the TK girdle would reduce the AGS cut-grade to 1 regardless of the referenced charts. The TK girdle would also cause the GIA cut-grade to be VG, even if it had EX proportions. So, in response to the original post, it's not a candidate for AGS Ideal.

2. RE AGS "Gold" Charts: << See for yourself... http://www.octonus.com/oct/mss/gia-agspgs.phtml >>

The charts referenced in this thread are not applicable to the AGSL Platinum metric. They correspond to the simpler 2D Gold system. The Gold system was developed in 2008 for manufacturers who demanded a predictable 2D system (like GIA's) which would permit them to know the cut grade decisively before sending the diamond to AGSL. It's not in frequent use, but Gold reports do show up from time to time.

The AGSL Platinum cut-guides predict 61T 41.8PA 30.5CA to be AGS1 in light performance. Of course the ultimate 3D grade can only be determined when details of cut-consistency, 3D optical precision and brillianteering are known. But, in general, the Platinum cut-guides are a best-case scenario.

3. Simulations-for-fun.

Since GIA rounds primary measurements I thought I'd check out the scenarios in DiamCalc.

Here are a few graphics showing the predicted AGSL Platinum grade and corresponding ASET simulation, and the current GIA grade and corresponding Ideal-Scope... Of course GIA does not employ reflectors, but the IS has a strong correlation to the HCA score, so I thought it might be interesting.

AGS 1 | ASET


GIA VG | Ideal-Scope


Bear in mind that these are "perfect" wire-frame simulations, not reflecting all 57 facets of this diamond... Just for fun.

General comments, for cut nerds like me:

The girdle is not a "bad" thick at all. The diamond has fine spread for weight thanks to the shallow depth. The pavilion angle is severe, which demands a notably shallow crown angle to avoid leakage, but some interesting choices saved the performance potential in an original piece of rough which I imagine was challenging.

Depending on cut consistency this should be a very bright diamond. There is no significant leakage. The deep PA could have caused deadly table-reflection (see the two rings of blue in ASET) but the cutter made two choices to counteract that. First was the long lower-halves (85%) which narrow the pavilion mains to reduce their contrast footprint. Second is the shallow crown height which was critical considering the deep PA. This saves brightness overall, but it reduces CH to 11.5% (as a reference, Tolk combos run 14-15+%).

If cut with strong consistency and a high level of optical precision the result is a very bright diamond. The potential to see fire is logically reduced. The virtual facets will also be smaller as a function of the details above. These things, combined with the unique contrast pattern will result in a different 'look' to the scintillation character (compared to an optically precise Tolk diamond), but only different... Not less attractive.

... there are fewer combos that make the AGS top grade than GIA's top grade. There are some combos that are in AGS top grade that does not qualify for GIA EX...

Just so. To elaborate, the center of GIA's system is deeper than the AGSL or the HCA. Put simply, there are AGSL Ideal combinations with pavilion angles of 40.6 and below which receive GIA VG-. And there are many GIA EX combinations with pavilion angles of 41.0 and higher which fall outside of AGS 0.

I hope this post contributes interesting information to the thread.

 

[diamondseeker2006]

Excellent as always, John!

 

[arkieb1]

Big thank you to John - that was amazing!!!!

I guess the basic answer is, I have seen triple ex hearts and arrows stones that should according to HCA and AGA be stunning stones BUT for a variety of reasons (strong fluoro, graining issues etc) look quite ordinary and stones in the VG category that have worse scores that were quite bright and stunning. These tools are great general indicators of predicting what a stone will be like. They are not hard and fast for every single stone.

Having said that if you compare your stone to a Cut above, A Brian Gavin Signature or a Crafted by Infinity will yours look as good - the simple answer is to most people probably not.

If you compare your stone to a generic triple ex stone will it look as good? Depends, possibly yes it could, depending upon the other stone and as John has pointed out, what YOUR visual preference is when looking at stones. A skilled cutter is trying to get as much weight out of the rough he started with and also changes the angles up or down to compensate for what they have to work with and to get a balance of the heaviest and the best looking stone possible.

Your stone has a bigger table and is not as deep as what I would personally pick for example, but that doesn't make it a bad stone, in fact many jewellers recommend this type of stone because it faces up bigger to the eye on your hand than one with a smaller table. The fact it has no leakage and you can see, thanks to John a nice red ASET image also means it will be a really bright diamond with heaps of fire.

The fact YOU love it, is all that really counts, don't worry about tools they are just that, a great set of general indicators, nothing more.

 

[The sun is shining]

Excellent as always, John!

Here, here! Perhaps the most informative thing I have read in ages!