What sins can cutters hide in these proportions?

[FB.]

Cutters are infamous for their ingenuity of maximising carat weight and still obtaining high cut grades - at the expense of beauty, if need be.

So, what can a cutter do to the following, fairly typical, "ideal" cut parameters, that would impair performance?

GIA certification
Round
6.5mm diameter x 4.0mm depth
1.0 (ish) ct
Cut grade: Ex
Polish/symm: Ex
Table: 56%
Crown angle: 34.5''
Crown height: 15.5%
Star: 55%
Girdle: med-sl.thick (faceted) 3.5%
Total depth: 61.5%
Pavilion angle: 40.8''
Pavilion depth: 43.0%
Lower half: 75%

Is it possible for a cutter to manage such excellent "on paper" proportions, yet produce a mediocre stone - and manage to slip it past the ideal grade for certification?

Thanks,

 

[FB.]

I guess that the lack of replies suggests that a GIA-cert stone, with proportions extremely close to the above, has very little chance of being a bad stone.

 

[Lorelei]

Date: 11/9/2009 7:10:18 AM
Author: FB.
I guess that the lack of replies suggests that a GIA-cert stone, with proportions extremely close to the above, has very little chance of being a bad stone.

Its pretty safe FB for a GIA proportioned diamond, even with rounding in either direction chances are it will be a well cut diamond. Even if the pavilion angle was slightly steeper than 40.8 and the angle averages weren't all that tight, there is still room for it to be a highly effective performer.

 

[John P]

Inconsistency: A stone with 8 pavilion angles ranging 40.2 - 41.4 would avg 40.8. Or CA. Or, in some instances, LH. Such a case would be rare and would certainly be revealed by IS or ASET.

Brillianteering: GIA dings stones to VG for >4 degrees (appx) painting or digging. While this largely serves to guard against issues, gamers could do damage w painting or digging of lower break facets to a lesser degree. Esp if paired with the inconsistency mentioned above. Again, ASET would typically reveal such a situation.

 

[jet2ks]

Date: 11/9/2009 11:22:28 AM
Author: John Pollard
Inconsistency: A stone with 8 pavilion angles ranging 40.2 - 41.4 would avg 40.8. Or CA. Or, in some instances, LH. Such a case would be rare and would certainly be revealed by IS or ASET.

Brillianteering: GIA dings stones to VG for >4 degrees (appx) painting or digging. While this largely serves to guard against issues, gamers could do damage w painting or digging of lower break facets to a lesser degree. Esp if paired with the inconsistency mentioned above. Again, ASET would typically reveal such a situation.

John, at what point would inconsistency of angles result in a downgrade in symmetry? How much variation could you have and still receive an Excellent grade?

 

[John P]

Date: 11/9/2009 11:33:17 AM
Author: jet2ks

Date: 11/9/2009 11:22:28 AM
Author: John Pollard
Inconsistency: A stone with 8 pavilion angles ranging 40.2 - 41.4 would avg 40.8. Or CA. Or, in some instances, LH. Such a case would be rare and would certainly be revealed by IS or ASET.

Brillianteering: GIA dings stones to VG for >4 degrees (appx) painting or digging. While this largely serves to guard against issues, gamers could do damage w painting or digging of lower break facets to a lesser degree. Esp if paired with the inconsistency mentioned above. Again, ASET would typically reveal such a situation.

John, at what point would inconsistency of angles result in a downgrade in symmetry? How much variation could you have and still receive an Excellent grade?

Hi Jet.

It depends. Remember that lab-graded symmetry is subjectively judged by the human eye at 10X magnification, not a 3D scan. As such it's aimed at detectable elements in facet symmetry (shaping & meet points) and proportion symmetry (table-culet off-center, wavy girdle etc). While PA and CA variance is perfunctorily addressesed downgrades occur only in extreme cases. As such, there are situations where variance can create leakage zones that go unaccounted-for in the symmetry grade; and in GIA's case the cut grade.

See this post for such an example.

 

[Rockdiamond]

Date: 11/8/2009 7:28:12 PM
Author:FB.
Cutters are infamous for their ingenuity of maximising carat weight and still obtaining high cut grades - at the expense of beauty, if need be.


Thanks,

HI FB,
All due respect, but infamous according to whom?
You''re making it out like there''s some vast conspiracy.
Is the GIA in on this as well?

 

[jet2ks]

Date: 11/9/2009 11:44:50 AM
Author: John Pollard

Date: 11/9/2009 11:33:17 AM
Author: jet2ks

Date: 11/9/2009 11:22:28 AM
Author: John Pollard
Inconsistency: A stone with 8 pavilion angles ranging 40.2 - 41.4 would avg 40.8. Or CA. Or, in some instances, LH. Such a case would be rare and would certainly be revealed by IS or ASET.

Brillianteering: GIA dings stones to VG for >4 degrees (appx) painting or digging. While this largely serves to guard against issues, gamers could do damage w painting or digging of lower break facets to a lesser degree. Esp if paired with the inconsistency mentioned above. Again, ASET would typically reveal such a situation.

John, at what point would inconsistency of angles result in a downgrade in symmetry? How much variation could you have and still receive an Excellent grade?

Hi Jet.

It depends. Remember that lab-graded symmetry is subjectively judged by the human eye at 10X magnification, not a 3D scan. As such it''s aimed at detectable elements in facet symmetry (shaping & meet points) and proportion symmetry (table-culet off-center, wavy girdle etc). While PA and CA variance is perfunctorily addressesed downgrades occur only in extreme cases. As such, there are situations where variance can create leakage zones that go unaccounted-for in the symmetry grade; and in GIA''s case the cut grade.

See this post for such an example.

Thanks John,

I understand the point and have seen other examples such as the one in the thread. Maybe my question is better phrased as: How much variation does there have to be in the angles before it is noticeable to the grader at 10x magnification? With too much variation, you have to start seeing some differences in proportion and facet size & shape between similar facets, right?

 

[Todd Gray]

A cutter can hide a lot within this range of proportions if the vendor / customer does not look past the proportions data provided on the lab report which is the result of the average measurements of each section. In fact, it was abuse of the initial AGS proportions rating system by the cutters that led to the AGS implementing their Light Performance rating system in June of 2005.

An average Crown Angle of 34.5 degrees can be the result of a minimal variance, such as a high measurement of 34.8 degrees and a low of 34.2 degrees; or it can be the result of a much broader variance such as a spread of something like 33.5 degrees up to 35.5 degrees; and the result would still be an average of 34.5 degrees... The same concept holds true for the Pavilion Angle, which is why it is important to look past the average measurements provided on the lab reports and obtain a computerized proportions analysis such as Sarin / OGI / Helium, indicating the facet-by-facet structure of the diamond to see how it flows...

Back in 2004, immediately following a spike in the price of diamond rough, I received two parcels of diamonds from different cutters, each business contained about 40 "ideal cut" diamonds and all of the diamonds were rejected because the crown angle measurements looked like this:

35.5 / 33.5 / 35.5 / 33.5 / 35.5 / 33.5 / 35.5 / 33.5

While these measurements resulted in an average of 34.5 degrees, they also resulted in the upper girdle facets looking dark... This makes a one carat diamond look like a ninety pointer

Needless to say, we rejected the diamonds and told the cutters that we wouldn''t sell slop! One of them responded by telling me that they had to cut this way in order to remain profitable as a result of the rough increase and that "if you drive a fruit cart, you need to buy fruit in order to sell it!" and I said "well we don''t sell rotten fruit!" and that pretty much ended our relationship...

Anyway, this is an example of "the sins" a cutter can hide in these proportions and why tools such as the ASET, Ideal Scope and H&A scopes are so important.

 

[Rockdiamond]

HI all!

Todd- I''m a little unclear about what happened. Did you look at these stones and reject them based on how they looked?
Or was it statistical data that caused you not to like the diamonds?
It also sounds as though the cutter was unreasonable, and a good person NOT to buy from.
I would be totally turned off as well to be pushed into buying something we did not like to help a cutter get rid of stones I found to be undesirable..

Luckily, we don;t run into that very often....actually at all.

 

[FB.]

Date: 11/9/2009 12:08:37 PM
Author: Rockdiamond

Date: 11/8/2009 7:28:12 PM
Author:FB.
Cutters are infamous for their ingenuity of maximising carat weight and still obtaining high cut grades - at the expense of beauty, if need be.


Thanks,

HI FB,
All due respect, but infamous according to whom?
You''re making it out like there''s some vast conspiracy.
Is the GIA in on this as well?

Conspiracy? No.
Just cutters endeavouring to make the most money out of their rough by retaining weight and still getting awarded an "excellent" cut grade. Most people would do the same. A few cutters - probably including the high-end-quality cutters on PS - might consider beauty rather than weight.

It seems as if a large proportion of GIA excellent graded stones are cut at the steep/deep end of the ranges (35.5/41.2) to maximise weight and profits, at the expense of beauty.

 

[FB.]

Date: 11/9/2009 1:46:16 PM
Author: Todd Gray
A cutter can hide a lot within this range of proportions if the vendor / customer does not look past the proportions data provided on the lab report which is the result of the average measurements of each section. In fact, it was abuse of the initial AGS proportions rating system by the cutters that led to the AGS implementing their Light Performance rating system in June of 2005.

An average Crown Angle of 34.5 degrees can be the result of a minimal variance, such as a high measurement of 34.8 degrees and a low of 34.2 degrees; or it can be the result of a much broader variance such as a spread of something like 33.5 degrees up to 35.5 degrees; and the result would still be an average of 34.5 degrees... The same concept holds true for the Pavilion Angle, which is why it is important to look past the average measurements provided on the lab reports and obtain a computerized proportions analysis such as Sarin / OGI / Helium, indicating the facet-by-facet structure of the diamond to see how it flows...

Back in 2004, immediately following a spike in the price of diamond rough, I received two parcels of diamonds from different cutters, each business contained about 40 ''ideal cut'' diamonds and all of the diamonds were rejected because the crown angle measurements looked like this:

35.5 / 33.5 / 35.5 / 33.5 / 35.5 / 33.5 / 35.5 / 33.5

While these measurements resulted in an average of 34.5 degrees, they also resulted in the upper girdle facets looking dark... This makes a one carat diamond look like a ninety pointer

Needless to say, we rejected the diamonds and told the cutters that we wouldn''t sell slop! One of them responded by telling me that they had to cut this way in order to remain profitable as a result of the rough increase and that ''if you drive a fruit cart, you need to buy fruit in order to sell it!'' and I said ''well we don''t sell rotten fruit!'' and that pretty much ended our relationship...

Anyway, this is an example of ''the sins'' a cutter can hide in these proportions and why tools such as the ASET, Ideal Scope and H&A scopes are so important.

Would those stones still obtain a GIA excellent for cut and symmetry?

 

[FB.]

Date: 11/9/2009 11:22:28 AM
Author: John Pollard
Inconsistency: A stone with 8 pavilion angles ranging 40.2 - 41.4 would avg 40.8. Or CA. Or, in some instances, LH. Such a case would be rare and would certainly be revealed by IS or ASET.

Brillianteering: GIA dings stones to VG for >4 degrees (appx) painting or digging. While this largely serves to guard against issues, gamers could do damage w painting or digging of lower break facets to a lesser degree. Esp if paired with the inconsistency mentioned above. Again, ASET would typically reveal such a situation.

Would such a wide range of angle (40.2-41.4) still get awarded excellent for symmetry?
Wouldn't GIA deduct a little from the symmetry and downgrade to very good or good?

 

[Rockdiamond]

Date: 11/9/2009 5:00:14 PM
Author: FB.

Date: 11/9/2009 12:08:37 PM
Author: Rockdiamond

Date: 11/8/2009 7:28:12 PM
Author:FB.
Cutters are infamous for their ingenuity of maximising carat weight and still obtaining high cut grades - at the expense of beauty, if need be.


Thanks,

HI FB,
All due respect, but infamous according to whom?
You''re making it out like there''s some vast conspiracy.
Is the GIA in on this as well?

Conspiracy? No.
Just cutters endeavouring to make the most money out of their rough by retaining weight and still getting awarded an ''excellent'' cut grade. Most people would do the same. A few cutters - probably including the high-end-quality cutters on PS - might consider beauty rather than weight.

It seems as if a large proportion of GIA excellent graded stones are cut at the steep/deep end of the ranges (35.5/41.2) to maximise weight and profits, at the expense of beauty.

Where I disagree here is that what you are calling "Steep Deep" was considered beautiful by a large sample of actual people looking- which is part of how they got included in GIA''s EX cut grade.
Will everyone like them?
No.
Will everyone like a stone chosen based on light return /ASET/ HCA?
No.
There is a range of proportions that people find pleasing.
It sounds like people here are saying GIA does not know what it''s doing- or can''t be trusted.

It''s very important to remember that wholesale buyers of diamonds are generally particular if they are paying for well cut goods- they look at what they are buying.
A cutter producing stones that look badly- or small for their weight- won''t be successful selling fine goods over the long run.

 

[FB.]

But weren't the GIA study-stones graded unmounted, with light able to "reverse-leak" into the bottom of the stone and out of the top?
From what I've seen, the borderline steep-deep (35-35.5/41-41.2) within the "excellent" grade are good stones, but the distinctly steep-deep would be better described as "very good" cut.

Let's face it; a steeper crown adds a bit more weight.....as does a slightly steep pavilion. Resulting in a few points extra weight retention and more stones passing magic weight numbers.
All of which is good for the cutter's profit margins, to the point where there would be a big incentive to cut stones to 35.5/41.2 proportions, rather than Tolkowsky's 34.5/40.7 ideal proportions (also supposedly partly derived from observations).

 

[Dancing Fire]

Date: 11/9/2009 1:46:16 PM
Author: Todd Gray
Back in 2004, immediately following a spike in the price of diamond rough, I received two parcels of diamonds from different cutters, each business contained about 40 ''ideal cut'' diamonds and all of the diamonds were rejected because the crown angle measurements looked like this:

35.5 / 33.5 / 35.5 / 33.5 / 35.5 / 33.5 / 35.5 / 33.5

While these measurements resulted in an average of 34.5 degrees, they also resulted in the upper girdle facets looking dark... This makes a one carat diamond look like a ninety pointer

i wouldn''t consider buying these kind of stone

too much variance.

 

[FB.]

Date: 11/9/2009 6:48:36 PM
Author: Dancing Fire

Date: 11/9/2009 1:46:16 PM
Author: Todd Gray
Back in 2004, immediately following a spike in the price of diamond rough, I received two parcels of diamonds from different cutters, each business contained about 40 ''ideal cut'' diamonds and all of the diamonds were rejected because the crown angle measurements looked like this:

35.5 / 33.5 / 35.5 / 33.5 / 35.5 / 33.5 / 35.5 / 33.5

While these measurements resulted in an average of 34.5 degrees, they also resulted in the upper girdle facets looking dark... This makes a one carat diamond look like a ninety pointer

i wouldn''t consider buying these kind of stone

too much variance.

But would such stones manage to qualify for the excellent cut grade and would they qualify for excellent symmetry?

 

[stone-cold11]

This is how GIA grades symm, http://www.gemology.ru/cut/english/symmetry/4.htm

So there is a possibility.

 

[John P]

Date: 11/9/2009 5:03:30 PM
Author: FB.

Date: 11/9/2009 11:22:28 AM
Author: John Pollard
Inconsistency: A stone with 8 pavilion angles ranging 40.2 - 41.4 would avg 40.8. Or CA. Or, in some instances, LH. Such a case would be rare and would certainly be revealed by IS or ASET.

Brillianteering: GIA dings stones to VG for >4 degrees (appx) painting or digging. While this largely serves to guard against issues, gamers could do damage w painting or digging of lower break facets to a lesser degree. Esp if paired with the inconsistency mentioned above. Again, ASET would typically reveal such a situation.

Would such a wide range of angle (40.2-41.4) still get awarded excellent for symmetry?
Wouldn't GIA deduct a little from the symmetry and downgrade to very good or good?

How to answer? Trying to spot such variance by human eye is more demanding than one might expect, and finish judgments can be subjective in the same manner as clarity. Your question depends on the human observer grading the diamond, where the variance occurs and if it's even observable to that person. It's not as simple as looking at a DC wireframe. Overall 3D shape and things like a tilted table influence the relative appearance or non-appearance of angular variance to the human eye. Even a Sarin scanner can be fooled by TT (link).

Having faith in GIA, I'd like to believe dramatic examples are noted and downgraded. From experience I can say that most diamonds cut with top angle combos do not (generally) show such dramatic variance. They happen with more frequency at the outer limits, although exceptions exist - which is why I remain a fan of doing OCD-analysis of any diamond; no matter the paper grades.

 

[yssie]

This is fascinating, from the newbie perspective, keep it coming

 

[yssie]

Date: 11/9/2009 6:50:59 PM
Author: FB.

Date: 11/9/2009 6:48:36 PM
Author: Dancing Fire

Date: 11/9/2009 1:46:16 PM
Author: Todd Gray
Back in 2004, immediately following a spike in the price of diamond rough, I received two parcels of diamonds from different cutters, each business contained about 40 'ideal cut' diamonds and all of the diamonds were rejected because the crown angle measurements looked like this:

35.5 / 33.5 / 35.5 / 33.5 / 35.5 / 33.5 / 35.5 / 33.5

While these measurements resulted in an average of 34.5 degrees, they also resulted in the upper girdle facets looking dark... This makes a one carat diamond look like a ninety pointer

i wouldn't consider buying these kind of stone

too much variance.

But would such stones manage to qualify for the excellent cut grade and would they qualify for excellent symmetry?

Question - would you see the result of this variation in the IS or ASET - what would it look like? I assume you would not see perfect H&A in that scope.