GIA vs. AGS cut grading?

[soxfan]

Wow! Thanks John! That was incredibly interesting! Thanks for posting it. I am by no means a cut expert, and I love learning more here every day.

I did compare it to a GIA ex cut and couldn't tell, so I feel lucky that I saved myself some money...

Thanks Arkieb1- I have to admit I was a little upset when I did the HCA but then I had to remind myself, like you said, it is just a predictor. I can't wait to get it back from Victor!!

 

[soxfan]

By the way, I didn't mention it before and I don't think it matters, but this was a RE-CUT. My jeweler had ordered it in, and then sent it out for re-cut and GIA certification....

 

[SB621]

Excellent as always, John!

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

+1 JOHN COME BACK AND POST MORE!

 

[John P]

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.

Good points. It may be useful to expand on the 'jeweler recommendation' you correctly mentioned, as it has to do with reflection:

The table facet is the largest on the diamond. Under intense lighting at the correct angle a bright mirror-like surface-reflection (called glare) will be returned to your eyes. Logically, the larger the table the larger the reflection.

It's a cool factor. But the claim that a diamond with -example- a 65% table will look 'bigger' than one of same diameter with a 55% table is only correct as it relates to surface reflection under certain lighting. Moreover, when that glare (reflection) is present it's difficult to distinguish fire (refraction) and contrast (obstruction). And most importantly, how large the diamond appears in normal and especially low-lighting depends on all proportions combined, not just the table area.

Example - with credit to Garry Holloway:
> The top photo shows examples cut at 6.00mm and 6.25mm in direct lighting.
> The bottom photo shows the same cut examples in lower lighting.
> Note how the larger spread (6.25 on the right) now appears smaller, as the proportions don't return light from edge to edge.
> To really see the difference - top to bottom photo - move several feet away from your computer monitor and look.




In a jewelry store showroom one might be inclined to consider them equal, or even choose the larger spread, as that light environment masks the fact that the edges will go dark in other environments.

So glare can be cool. And it's definitely in-use as a selling-point for diamonds with larger tables. But keep in mind that how a diamond appears away from showroom lights will always revert back to the non-negotiable and critical details of its total proportions and cut-consistency.

 

[John P]

By the way, I didn't mention it before and I don't think it matters, but this was a RE-CUT. My jeweler had ordered it in, and then sent it out for re-cut and GIA certification....

This makes complete sense. Recut success or failure often depends on preserving a value-related weight threshold. The cutter was able to "save" 2 cts here which I wager was the goal. No doubt he/she knew the proportions were going to be VG, so no harm in the girdle being TK in a place or two, since the spread is fine. Likely the most attractive 2+ carat outcome.

 

[John P]

Excellent as always, John!

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

+1 JOHN COME BACK AND POST MORE!

Diamondseeker, TSIS and SB621 - Thanks for the comments. They're appreciated.

 

[soxfan]

Excellent as always, John!

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

+1 JOHN COME BACK AND POST MORE!

YES!!!!!! This information was awesome.

 

[Garry H (Cut Nut)]

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

I noted in a later post that the stone was recut.
Given that there is still 0.04ct to play with, the optimum result would have been to reduce the crown angle a bit and get a smaller table (29 degrees, which raises the risk of chipping, but the girdle is fairly heavy - and table size would then be 58-59%).
This would lift the life of the stone a little.
Also at outlying proportions like these there is a lot of impact from the lower girdle length and some variances caused by star proportions.
It is already likely to be very 'bright' but slightly lacking in contrast.

 

[Paul-Antwerp]

Last week, a bigger cutting-house asked us to study their newly finished GIA-stock and to give advice on optimizing it.

To start, we looked at their GIA-3EX stones between 1.00 and 2.50, and selected 30 stones that, based upon average proportions, could be AGS-Ideal. We ran a Sarin on these 30, and took that through the AGS cut-grading-software.

Of the 30, only 6 were actual AGS-Ideals and 3 of them got a final AGS-cut-grade of AGS-3.

I think this clearly shows that using proportion-charts to predict AGS-grades is far from correct.

Thinking that I personally consider the top-AGS-grade still far too wide then becomes mind-boggling.

Live long,

 

[arkieb1]

Good points. It may be useful to expand on the 'jeweler recommendation' you correctly mentioned, as it has to do with reflection:

The table facet is the largest on the diamond. Under intense lighting at the correct angle a bright mirror-like surface-reflection (called glare) will be returned to your eyes. Logically, the larger the table the larger the reflection.

It's a cool factor. But the claim that a diamond with -example- a 65% table will look 'bigger' than one of same diameter with a 55% table is only correct as it relates to surface reflection under certain lighting. Moreover, when that glare (reflection) is present it's difficult to distinguish fire (refraction) and contrast (obstruction). And most importantly, how large the diamond appears in normal and especially low-lighting depends on all proportions combined, not just the table area.

So glare can be cool. And it's definitely in-use as a selling-point for diamonds with larger tables. But keep in mind that how a diamond appears away from showroom lights will always revert back to the non-negotiable and critical details of its total proportions and cut-consistency.[/quote]

OMG that is so cool - I hope you just made things clear for new buyers because I see so many people buy stones that look great in the showroom under controlled lighting and then when them get them home they morph into not exactly what I would call dogs as such, but they lose that brilliant glare you are describing......