How Deep is GIA

[Houseparty]

Hi PS,
We have seen numerous critiques of GIA''s 2006 Cut Grade. One of them is that GIA cut grade "Excellent" includes too many steep/deep combinations.
Here is my question; How deep could they actually be?

For example, if a stone had the following properties:
55 Table
35.0 degree CA
41.6 degree PA
Slightly Thick Girdle (which I have learned is about 4.2% at the mains)
Pointed Culet
finally, make any normal assumptions for data not provided...

This stone should plot as a GIA "Ex", but what would be the total depth?
63.0%, 64.0%???

I tried to find this and calculate it myself with no luck. How about one of you smart guys?

Thanks,
David

 

[JohnQuixote]

Date: 4/18/2006 6:41:20 PM
Author:Houseparty
Hi PS,
We have seen numerous critiques of GIA''s 2006 Cut Grade. One of them is that GIA cut grade ''Excellent'' includes too many steep/deep combinations.
Here is my question; How deep could they actually be?

For example, if a stone had the following properties:
55 Table
35.0 degree CA
41.6 degree PA
Slightly Thick Girdle (which I have learned is about 4.2% at the mains)
Pointed Culet
finally, make any normal assumptions for data not provided...

This stone should plot as a GIA ''Ex'', but what would be the total depth?
63.0%, 64.0%???

I tried to find this and calculate it myself with no luck. How about one of you smart guys?

Thanks,
David

Hey David

GIA won''t allow EX for over 63% in any cases of which I am aware.

The problem is not strictly depth % It''s the combinations allowed within that 63%. A diamond can be steep/deep at different total depths.

 

[valeria101]

That''s a funny way of describing a diamond (no depth, no girdle)! Where do the numbers come from?


Btw, the ''steep & deep'' wording refers of crown and pavilion angles. The total depth of the diamond doesn''t have to be deep.


From the numbers you''ve give, it is the thickish girdle that makes it deeper (and your guess 63-64% deep is as good as mine - about 63.4 or so...). Make girdle width to a thin-medium (below 2%) for the same table and angles and that''s not a deep diamond anymore. Still, the angles get a bit near that ''steep deep'' category - because either 41 degrees pavilion would ''do better'' with a less steep crown, or the 35 degrees crown would do better with a less steep pavilion angle... as far as I can tell

Besides, this may not be the best round diamond GIA has ever graded ''Excellent''... but whatever those numbers tell, doesn''t sound like such a horrible disaster either. If the price is great... and the diamond looks nice in person (after all, the numbers do not tell me about symmetry, and it may be that rounding and measurement imprecision just made this borderline case fall on the bright side etc.).

My 2c.

 

[Houseparty]

Date: 4/18/2006 8:04:23 PM
Author: valeria101


That''s a funny way of describing a diamond (no depth)! Where do the numbers come from?


Btw, the ''steep & deep'' wording refers of crown and pavilion angles. The total depth of the diamond doesn''t have to be deep.

Hi Valerie,
Thanks for your reply, this is a hypothetical situation (although such stones certainly do exist). That is why it is a "funny way to desribe a diamond".

Essentially, I am curious how deep can a stone be and still get GIA''s "Ex" Cut Grade? John from WF replied that the new GIA Cut Grade System has a cap at 63.0% total depth. Since / If that is the case, the question is technically moot, and the answer is simply 63%. So thank you John, I never saw that... I also understand your distinction that the key issue is between angle combos not total depth.

However, out of intellectual curiosity, what would be the total depth of the diamond at the top of this thread? I believe there is a mathematical answer, I just don''t know how to calculate it.

Sincerely,
David

 

[JohnQuixote]

Date: 4/18/2006 8:43:35 PM
Author: Houseparty
Hi Valerie,
Thanks for your reply, this is a hypothetical situation (although such stones certainly do exist). That is why it is a 'funny way to desribe a diamond'.

Essentially, I am curious how deep can a stone be and still get GIA's 'Ex' Cut Grade? John from WF replied that the new GIA Cut Grade System has a cap at 63.0% total depth. Since / If that is the case, the question is technically moot, and the answer is simply 63%. So thank you John, I never saw that... I also understand your distinction that the key issue is between angle combos not total depth.

However, out of intellectual curiosity, what would be the total depth of the diamond at the top of this thread? I believe there is a mathematical answer, I just don't know how to calculate it.

Sincerely,
David

David, going by the numbers the diamond above, with a slightly thick girdle, could range into the high 63%s. Bear in mind that the way the labs arrive at girdle min-max and averages is different so the 4.2% you reported would need to be about halved to be reported as slightly thick in the GIA metric (2.1%).

The AGS notes thickest and thinnest places at any point on the girdle. GIA reports the thickest and thinnest parts measured at valleys only. Therefore girdle notations on AGS reports will seem to have much more variation.

 

[Houseparty]

Hi John,
Thank you for your thoughtful reply. I attended a GIA Cut Grade Seminar last night, and sure enough, there was a chart in the "manual" that capped GIA EX cut at 63.0% just like you said. What threw me off is that Rhino posted a GIA Facetware screen shot on another thread in the past that had a GIA EX with a 63.2% depth. Perhaps this is another example of a GIA''s inconsistency.

One thing I did learn, is that my initial premise on this thread may not have been valid. Although a 55 table, 35 CA, 41.6 PA, is still EX, and slightly thick girdle is EX, these factors combined may not produce a EX; that how we get the millions of combinations...

Thanks again for your help.

Sincerely,
David