GIA Excellent - Steep/Deep

[FB.]

This seems to be one of the most common sets of proportions - even for stones which aren''t desperately trying to cling to a magic carat number.

A typical overall set of proportions for a GIA-Ex would be something like:
Table 57%
Crown 15% (35'')
Star: 50%
Girdle: medium
Pavilion: 44% (41.2'')
Lower half: 80%

Such stones score about 4 on the HCA; good or very good in most categories.
I know that many PS-ers dislike the steep-deep stones (with reduced light return under the table), but someone must be buying them.

Having seen (and owning), for myself, a number of these stones, you have to get the light and position fairly consistent (from straight above) to barely be able to notice the darker table - and the darker table is often masked by the appearance of the black arrows, if the symmetry is excellent.

But is the fear of a slightly darker table generally worse than the reality ? - since most diamonds get viewed from avariety of angles, whereas the HCA seems to have been designed for evaluating the directly-overhead view (correct me if I''m wrong).

Regards,

 

[stone-cold11]

The longer lower half is helping to mitigate the larger pavilion angle, also depending on the round of numbers by GIA, if the lower half number is round down to 80% while the pavilion angle is actually rounded up to 41.2 degrees, that might probably not even have leakage there with good symm.

Most likely the stereoscopic effect of our binocular vision is kicking in here too. But the overall light return will still be less if there is leakage and you will probably be paying for a slightly more weight that is not making the stone perform better so why pay for that?

 

[Lorelei]

Date: 10/20/2009 6:14:01 AM
Author:FB.
This seems to be one of the most common sets of proportions - even for stones which aren't desperately trying to cling to a magic carat number.

A typical overall set of proportions for a GIA-Ex would be something like:
Table 57%
Crown 15% (35')
Star: 50%
Girdle: medium
Pavilion: 44% (41.2')
Lower half: 80%

Such stones score about 4 on the HCA; good or very good in most categories.
I know that many PS-ers dislike the steep-deep stones (with reduced light return under the table), but someone must be buying them.

Having seen (and owning), for myself, a number of these stones, you have to get the light and position fairly consistent (from straight above) to barely be able to notice the darker table - and the darker table is often masked by the appearance of the black arrows, if the symmetry is excellent.

But is the fear of a slightly darker table generally worse than the reality ? - since most diamonds get viewed from avariety of angles, whereas the HCA seems to have been designed for evaluating the directly-overhead view (correct me if I'm wrong).

Regards,

Hi FB

With some steep deeps a longer lower girdle facet such as 80% can sometimes help steep deep stones by offsetting leakage, so it is possible some SD's will look worse than others depending on the length of the LGF's and overall tightness of the cut proportions. As to the reality of how these look, I can only go by the experiences I have read of some others with steep deep stones. An example is Kelli, she not only noticed the effects of her steep deep but it really bothered her with the leakage she could see, that she had the diamond recut by Brian Gavin and the result is kicken - to quote Karl! Also GIA round the numbers as you know so depending on which way they are rounded ( if the proportions are in fact shallower than quoted there might not be much of a problem) if steeper there might be.

Moral of the story though - even with steep deeps with longer lower girdle facets always get an Idealscope to check for leakage and do not assume longer LGF's will automatically save it.

You might enjoy the video on this page, Surveying Diamonds in NY. If I remember rightly it is our previous admin Leo and Jim Schultz from JA asking passers by what they thought of different angled diamonds to include a steep deep in Central Park.

 

[BobR]

Hi
I''m new to the forums and a relative novice to the science around cut. I''m about to buy 3 GIA cert stones for a pendant that seem to falll in this steep/deep category
Specs are
Stone 1 (0.65ct) Stone2(0.5ct) Stone 3 (0.35ct) AGS Ideal (7.5 ct see notes below)
table 57 57 57 57.1
depth 61.3 61.8 61.9 62.6
crown angle 35 34.5 36 34.9
height 15 15 15.5 15.0
pav angle 41.0 41.4 41 41.3
pav depth 43 44 43.5 43.9
cut grade Ex Ex Ex Ideal (0)
polish Ex Ex VG Ideal (0)
symm Ex Ex VG Ideal (0)
Star/Lower half 55/75 55/75 50/80 48/80

We have some 3 stone GIA cert earrings which generally have smaller crown angles and proportions whiich, based on facetware / ags proportion tables are GIA Ex/AGS ideal (or would be but for symm and polish are mostly good/good).


My questions are :-
1) should I expect inferior performance from these 3 stones than those in the earrings?
2) What really cofuses me is browsing Internet I found a AGS Ideal0 which also seems to fall into steep/deep
see spec above. This is AGS cert 0010332004 and is a big stone 7.552ct. It scores 4.3 on HCA. So are steep/ deep excellents a problem or not - or are considerations different for large stones??
Thanks for any insights
Bob
Bob

 

[stone-cold11]

Looks like another cutter managed to game AGS''s system.

 

[stone-cold11]

Date: 10/27/2009 5:34:16 PM
Author: BobR

My questions are :-

1) should I expect inferior performance from these 3 stones than those in the earrings?

2) What really cofuses me is browsing Internet I found a AGS Ideal0 which also seems to fall into steep/deep

see spec above. This is AGS cert 0010332004 and is a big stone 7.552ct. It scores 4.3 on HCA. So are steep/ deep excellents a problem or not - or are considerations different for large stones??

Thanks for any insights

Bob

Actually leakage in smaller stones are less detectable.

Better to find a better proportioned stone. Stone one will have the best performance.

 

[BobR]

Thanks for reply, good to know I shouldn''t have much visible leakage given the size of stones but am wondering -
Is there a specific miinimum crown/pav angle combination for steep/deep and what about mixed combos such as my 3 stones
Im guessing that stone 1 is not in fact a steep/deep in any sense? Is this right?
but what about stone 2 34.5/41.4 and stone 3 inverse 36/41. From what I''ve read the steep/deep categories apply to non compensating mixed combos as well?? i.e. although stone 2 has 41.4 pav angle there''d be no issue if crown angle were 34 or less? Similar if stone 3 had a 40.6 pav angle to compensate for the 36 crown??
Also the AGS gets an ideal light perf with 34.9/41.3 - this seems to contradict perceived wisdom and also is at odds with its 4.3 hca score??
Bob

 

[stone-cold11]

Date: 10/27/2009 8:16:32 PM
Author: BobR
Thanks for reply, good to know I shouldn't have much visible leakage given the size of stones but am wondering -

Is there a specific miinimum crown/pav angle combination for steep/deep and what about mixed combos such as my 3 stones

Im guessing that stone 1 is not in fact a steep/deep in any sense? Is this right?

but what about stone 2 34.5/41.4 and stone 3 inverse 36/41. From what I've read the steep/deep categories apply to non compensating mixed combos as well?? i.e. although stone 2 has 41.4 pav angle there'd be no issue if crown angle were 34 or less? Similar if stone 3 had a 40.6 pav angle to compensate for the 36 crown??

Also the AGS gets an ideal light perf with 34.9/41.3 - this seems to contradict perceived wisdom and also is at odds with its 4.3 hca score??

Bob

Stone 1 is on the edge, so could be good or some slight leakage. No way to really tell without an Idealscope image. #2 and #3 stone, yap, CA 34.5 is good combination with PA 40.8, CA 36.0 combination with PA 40.6. But #3 stone will need an Idealscope image as that PA might have probably obstruction issue.

As I said, the AGS stone is gamed. The stone will have leakage under table, probably used that to provide control leakage for the scintillation effect as there will be no contrast/shadow zone. The stone will definitely look less bright than other AGS0 stone. An IS/ASET image will tell more.

 

[John P]

Date: 10/27/2009 8:52:24 PM
Author: Stone-cold11

As I said, the AGS stone is gamed. The stone will have leakage under table, probably used that to provide control leakage for the scintillation effect as there will be no contrast/shadow zone. The stone will definitely look less bright than other AGS0 stone. An IS/ASET image will tell more.

Actually I was surprised it made the grade, but then I saw the unique size. Wow.

The AGS cut guides predict AGS3 light performance for 57/41.3/34.9, but with something this large we're out of the ordinary and into the extremely rare. Rules for common sizes may not apply. The 62.7%D makes me wince but if it was ray-traced and made the grade...

Still, I wish we'd had this rough. This cutter was so anxious to keep the saleable 7.5ct mark that he/she allowed extra depth in the pavilion, which takes it out of the scientific performance bullseye (and the 12.51 mm spread faces up like a 7.25ct modern Tolk in any case).

With a tiny (TINY) adjustment it could be blazing in all light conditions. Just recut the pavilion to 40.9 avg and, yes, you drop below 7.50 cts but the diamond loses NO spread, and check the difference...

41.3 Pavilion Angle, 7.55 ct

 

[John P]

40.9 Pavilion Angle, 7.48 ct

(Normal disclaimer: These are "perfect" wire-frame simulations which obviously do not reflect the cut consistency of the actual diamond - though I presume it's pretty decent, given the AGS grade.)

 

[stone-cold11]

Ya, weird. Would like to see the actual ASET and see how far AGS bends over backwards for that big a stone.

 

[Lorelei]

Date: 10/27/2009 9:13:28 PM
Author: John Pollard

Date: 10/27/2009 8:52:24 PM
Author: Stone-cold11

As I said, the AGS stone is gamed. The stone will have leakage under table, probably used that to provide control leakage for the scintillation effect as there will be no contrast/shadow zone. The stone will definitely look less bright than other AGS0 stone. An IS/ASET image will tell more.

Actually I was surprised it made the grade, but then I saw the unique size. Wow.

The AGS cut guides predict AGS3 light performance for 57/41.3/34.9, but with something this large we''re out of the ordinary and into the extremely rare. Rules for common sizes may not apply. The 62.7%D makes me wince but if it was ray-traced and made the grade...

Still, I wish we''d had this rough. This cutter was so anxious to keep the saleable 7.5ct mark that he/she allowed extra depth in the pavilion, which takes it out of the scientific performance bullseye (and the 12.51 mm spread faces up like a 7.25ct modern Tolk in any case).

With a tiny (TINY) adjustment it could be blazing in all light conditions. Just recut the pavilion to 40.9 avg and, yes, you drop below 7.50 cts but the diamond loses NO spread, and check the difference...

41.3 Pavilion Angle, 7.55 ct

''Zactly!!!

 

[Karl_K]

Date: 10/27/2009 5:38:24 PM
Author: Stone-cold11
Looks like another cutter managed to game AGS's system.

very carefully paint the pavilion to avoid the downgrade, kill some optical symmetry and you made an ags3 into a 0 grade without the performance of a real AGS0.
I am just surprised it took this long for the games to begin.
My guess is that because it is far easier to game GIA EX and there is much more gain to gaming the GIA system for most to bother.

 

[John P]

I wouldn't be surprised if it had a GIA report to begin with and was sent to AGS by request at some point Karl.

Speculation I know, but this is a rare honker. I see such requests honored with no questions for 5 cts smaller.

 

[Karl_K]

Date: 10/28/2009 10:38:44 AM
Author: John Pollard
I wouldn''t be surprised if it had a GIA report to begin with and was sent to AGS by request at some point Karl.


Speculation I know, but this is a rare honker. I see such requests honored with no questions for 5 cts smaller.

I would agree.

Either that they ran it through AGS pgs saw they had a lucky 0 and sent it off to AGS.
If it was a 1ct that would be more likely I think but at 7.5ct I would tend to think they knew exactly what they were doing.

 

[BobR]

Hi
The cert (with ASET) can be found on the following link

http://www.abazias.com/database/CertWin.asp?stock=70912621

I'd be interested on thoughts as to to how this stacks up against expectations given the ca/pa combo, and to what extent if any this can be assumed for specs with near identical proportionsj?

I'd also be interested in views on an earlier question - is there a specific ca/pa combo beyond which (On Average) noticeable leakage occurs???
eg 36/41.1? and can I extrapolate this to also equate for 35.5/41.2, 35/41.3 etc
Lorelei
If you read this - I haven't forgotten your request - its just that on this issue/point it was better to continue where left off!!
rgds
Bob

 

[FB.]

Bob

From what I've seen, 41.2 pavilion angle and 35.5 crown angle is the point at which you can start to see some darkness under the table.
Stones with 41.2/35.5 that have excellent symmetry and polish, or with clever cutting of the minor facet proportions, may not show darkness. I'd say that 41.2/35.5 was the most noticeable tipping point, but it's still a gradual loss of brilliance as the angles get less complementary.
(GIA work in 0.2 pavilion increments and 0.5 crown increments).

I think the relationship is something like:
The ideal crown angle is about 34.5 and the ideal pavilion angle is about 40.6.
An increase of 0.5 degrees on crown angle requires a decrease of 0.2 degrees on the pavilion angle to compensate.

Shallower crown angle offset by steeper pavilion angle makes the stone brighter (brilliant ideal cut). e.g 33.0 crown / 41.2 pavilion.

Steeper crown angle and shallower pavilion angle makes the stone more fiery (fiery ideal cut). e.g. 36.0 crown / 40.0 pavilion.

 

[BobR]

Thanks for this
Interesting to see what my 3 pendant stones look like when they arrive
1) 36/41 GIA Ex/VG/VG should equate roughly to tipping point
2) 34.5/41.4 GIA triple Ex should be just inside
3) 35/41 should be well inside (GIA triple ex andI guess possible AGS Ideal)
As an aside Good old Gold has an interesting comparison between GIA triple Ex (Steep/Deep 35.1/41.15 vs AGS Ideal 34.8/40.7 with painted lower girdle, and despite all technology pointing to the AGS., the human preference was unanimous (although a small sample) in favour of the GIA Ex
.

 

[BobR]

Link to GoG article

http://www.goodoldgold.com/4Cs/NewCutGrading/GIAExAGSIdeal/

 

[Garry H (Cut Nut)]

Date: 10/27/2009 9:13:28 PM
Author: John Pollard

Date: 10/27/2009 8:52:24 PM

Author: Stone-cold11


As I said, the AGS stone is gamed. The stone will have leakage under table, probably used that to provide control leakage for the scintillation effect as there will be no contrast/shadow zone. The stone will definitely look less bright than other AGS0 stone. An IS/ASET image will tell more.

Actually I was surprised it made the grade, but then I saw the unique size. Wow.


The AGS cut guides predict AGS3 light performance for 57/41.3/34.9, but with something this large we''re out of the ordinary and into the extremely rare. Rules for common sizes may not apply. The 62.7%D makes me wince but if it was ray-traced and made the grade...


Still, I wish we''d had this rough. This cutter was so anxious to keep the saleable 7.5ct mark that he/she allowed extra depth in the pavilion, which takes it out of the scientific performance bullseye (and the 12.51 mm spread faces up like a 7.25ct modern Tolk in any case).


With a tiny (TINY) adjustment it could be blazing in all light conditions. Just recut the pavilion to 40.9 avg and, yes, you drop below 7.50 cts but the diamond loses NO spread, and check the difference...


41.3 Pavilion Angle, 7.55 ct

Sad, really sad.
This stone would have dark zones in the table.
Once dirty it would suffer badly.
There are better large stones around.
It was once so that 3ct H&A''s was rare. Not so any more

 

[Karl_K]

Date: 10/28/2009 10:29:09 AM
Author: Karl_K

Date: 10/27/2009 5:38:24 PM

Author: Stone-cold11

Looks like another cutter managed to game AGS''s system.

very carefully paint the pavilion to avoid the downgrade, kill some optical symmetry and you made an ags3 into a 0 grade without the performance of a real AGS0.

I am just surprised it took this long for the games to begin.

My guess is that because it is far easier to game GIA EX and there is much more gain to gaming the GIA system for most to bother.

I nailed that one.....