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GIA cut grading -- VG vs EX

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enbcfsobe

Brilliant_Rock
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Jan 17, 2007
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Perhaps this is just coincidence, but in looking at the diamond selection, some in person and some online, (in the .95-1.3 ct range, F-G, VS1-VS2) of a nearby B&M and running the info through the HCA (not all had enough info), only one of the GIA "Excellent" cuts got a < 2 HCA rating, and most were around 4. In contrast, this jeweler''s GIA "Very Good" cut stones for which sufficient information was available were all < 3 on the HCA.

While I know that there are other factors to consider for light performance that are assumed as constants in the HCA, including girdle and symmetry, are these what GIA uses to downgrade from EX to VG? All the EX and VG cut grades I have looked at so far fall within the GIA EX box shown on the HCA graph.
Should I assume that these stones will be inferior for other reasons?

How bad is "Good" or "Very Good" symmetry or a "very thin - med" girdle? Should I expect that these things would cause leakage that might make them poor performers despite a good HCA rating? Do such VG graded stones warrant a return-trip with an Ideal Scope (waiting for delivery of mine)?

Another issue is older stones with no cut grade that a vendor lists as "Ideal" (interestingly, only one had more than a "good" hca score). If they score well on the HCA are they worth a closer look? Do you ask the vendor to send them for regrading?

I''m curious, and also trying to evaluate whether the additional time is worth it, esp. since another PSer reported that this B&M, after some negotiation, came close to matching prices of PS vendors.

Maybe this is just an example of the limits of HCA and/or the imposition of additional criteria by GIA.


The most interesting stones were:

1.04 G VS1 GIA (2005, no cut grade -- angle info from GIA lookup)
6.52-6.58X3.93
vg/vg/no
med-sl thk
Depth: 60.0%
Table: 59.0%
Crown Angle: 35.5
Pav. Angle: 40.4
[1.1 HCA -- ex in all categories]

1.20 G VS1 GIA VG cut
6.77-6.82X4.24
ex/g/no
Depth: 62.4%
Table: 54.0%
Crown Angle: 33
Pav. Angle: 41
[0.9 HCA -- vg in spread, ex all others]
 

Garry H (Cut Nut)

Super_Ideal_Rock
Trade
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Aug 15, 2000
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15,141
HCA is a ''one eyed view'' - because it is my view, and also because when I did it - I did it with a one eyed face up view as most important.

Arguably with 2 eyes I should e less strict on slightly steep deep leaky diamonds that GIA includes - say 41.2P 34.5C. - see this image here - this steep deep stone shows under the table leakage face up - but it disappears when seen with 2 eyes - it is a photo shop attempt only to show the effect

But GIA is wrong to allow very steep deep stones and they are way off with their shallow boundary as explained here https://www.pricescope.com/community/threads/gia-diamond-dock-simple-summary.42538/

63.2 combined Small33.jpg
 

enbcfsobe

Brilliant_Rock
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Jan 17, 2007
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1,154
Wow. Maybe I''ve overreached in terms of my understanding of what is happening with regards to cut and performance. That thread was helpful, though a bit over my head.
Is the bottom line that GIA VGs may, in some cases, perform better than some of the EXs?
Will I be able to tell which of either are good using an IS?

Particularly since I''m looking at a fairly modest size and would mauch rather see my BF pay less for a larger looking stone than vice versa, so I''m not crazy about the deeper stones regardless of performance (though it does seem true that they show favorably in store lighting). How do I tell when shallow is too shallow?

Still not sure what the effects of vg or g in polish or symmetry are...

Confused but still curious,
enbcfsobe
 

JulieN

Super_Ideal_Rock
Premium
Joined
Jul 25, 2005
Messages
13,324
Date: 1/24/2007 5:03:17 PM
Author: enbcfsobe
Wow. Maybe I''ve overreached in terms of my understanding of what is happening with regards to cut and performance. That thread was helpful, though a bit over my head.
Is the bottom line that GIA VGs may, in some cases, perform better than some of the EXs?
yes
Will I be able to tell which of either are good using an IS?
yes

Particularly since I''m looking at a fairly modest size and would mauch rather see my BF pay less for a larger looking stone than vice versa, so I''m not crazy about the deeper stones regardless of performance (though it does seem true that they show favorably in store lighting). How do I tell when shallow is too shallow?
usually when I start seeing some extra black under IS, or "paddles"

Still not sure what the effects of vg or g in polish or symmetry are...
if you have an Ideal Scope, you can check it out yourself

Confused but still curious,
enbcfsobe
 

enbcfsobe

Brilliant_Rock
Joined
Jan 17, 2007
Messages
1,154
Thanks, Julie! I got my IS in the mail today, so now I will be able to see for myself!

In terms of the effects of less-than excellent polish/symmetry, where should I expect that to manifest in the IS image? Will that affect the symmetry of the image (ie how well the pieces of the arrows line up), or will it change the shades of color I am seeing, or both?

It seems like the answer is that I may find stones within both EX and VG that have both a good HCA and IS image, and should therefore look at both and weed out based on those two things, instead of sticking just to the EX/VG (and get more advanced info from appraisal before making final purcahse)?
 

JulieN

Super_Ideal_Rock
Premium
Joined
Jul 25, 2005
Messages
13,324
arrows are usually pretty decent. don''t see much of the splintery look anymore.

don''t expect H&A perfection.

i think the thing to look out for with symmetry is leakage under the table on one side of the stone.
 

enbcfsobe

Brilliant_Rock
Joined
Jan 17, 2007
Messages
1,154
Will that make it look more light pink on one side than the other? Or will there be more white spots around the edges on one side?

I don't expect perfect arrows -- even the ACA ones don't look precisely symmetrical to me on the IS images.

I'm still very new to the IS -- I've only gotten to look at the CZ so far (alas, I just don't have all sorts of loose diamonds lying about the house).
I thought the IS would have instructions...back to their website I go!
 

enbcfsobe

Brilliant_Rock
Joined
Jan 17, 2007
Messages
1,154
Thanks. So it is both pink-er overall and there''s greyish-white showing around the middle on the top half.

*sigh* starting to think that maybe ignorance was bliss. so many things to think about... buying online is looking better to me every day. with this learning curve, i''ll be lucky to find something before memorial day!!
 
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