Letter to the Editor of the Australian Gemmologist

[Garry H (Cut Nut)]

Date: 3/12/2007 12:41:45 PM
Author: michaelgem
Have another look folks. Like Sergey said 'Take software such as PGS, and check different combinations of parameters, print it. You will have created a chart. Charts are just more effective for human analysis. Human analysis is very important for developing the cutter’s strategy.'

What can we learn about cutting strategy from this 57% table chart for a 6mm, 50%sl, 80%lg, 3.5%girdle optically symmetric diamond?

If I were a cutter I would be interested in knowing that this chart indicates that the center of the 'sweet spot' for Ideal 0 according to the AGS-PGS is 41.1, 34.25 not the AGS guideline charts center of 40.7, 35.

I would also be interested in knowing that the guideline indication that it is safe to cut Ideal 0 combinations of 40.2, 36.5 and 40.2, 37 is no longer true.

I would also be interested in knowing that I can cut a 41.6, 32.25 with decent leeway even though the guideline charts guide me away from that combination.

I would also be interested in knowing that I can cut a 40.5, 36.75 and get an Ideal 0 even though the guideline charts also guide me away from that combination.

Michael Cowing

It seems the idea that AGS parameters are wider than suggested by the use of AGS PGS software may be affected by a bug in the PGS software (whch according to Peter Yantzer is not a problem with their more robust AGSL version).

So this and other debate that raged here for a while may have been a waste of time.

I would ask those with the PGS software to check the grade of a 65% table
Pavilion: 41.4 Crown: 33 Lower girdle facet length: 80% or Lower girdle facet depth: 81.5% and Stars of : 55% and 49%

all with a Girdle bezel of 3%
I have attached the 55% star .dmc

 

[RockDoc]

Garry

I would be happy to run the PGS for the scan file, but inorder to run it on PGS it needs to be an .stl or .srn file.


I know that some other formats can be changed to work, but there seems to be "problems" in the accuracy of the scan when other formats are converted. So using other formats than the srn or stl files cause some problems with the vector points.

WIth the PGS software, it needs to have a scan of the stone, as you can''t just enter numbers into it like Dia Calc.

If you can get that scan file, I''d be happy to run it for you.

EXTRA NOTE: The PGS software has a pretty serious update, coming out in just a few weeks, where some of the bugs are corrected, and the emerald cut evaluation sets will be in it.

Perhaps AGSL has a more robust program, but Jim Caudill had told me when I bought it that it was the same as the lab used.

Rockdoc

 

[Serg]

Date: 3/31/2007 7:45:44 AM
Author: Garry H (Cut Nut)

Date: 3/12/2007 12:41:45 PM
Author: michaelgem
Have another look folks. Like Sergey said ''Take software such as PGS, and check different combinations of parameters, print it. You will have created a chart. Charts are just more effective for human analysis. Human analysis is very important for developing the cutter’s strategy.''

What can we learn about cutting strategy from this 57% table chart for a 6mm, 50%sl, 80%lg, 3.5%girdle optically symmetric diamond?

If I were a cutter I would be interested in knowing that this chart indicates that the center of the ''sweet spot'' for Ideal 0 according to the AGS-PGS is 41.1, 34.25 not the AGS guideline charts center of 40.7, 35.

I would also be interested in knowing that the guideline indication that it is safe to cut Ideal 0 combinations of 40.2, 36.5 and 40.2, 37 is no longer true.

I would also be interested in knowing that I can cut a 41.6, 32.25 with decent leeway even though the guideline charts guide me away from that combination.

I would also be interested in knowing that I can cut a 40.5, 36.75 and get an Ideal 0 even though the guideline charts also guide me away from that combination.

Michael Cowing

It seems the idea that AGS parameters are wider than suggested by the use of AGS PGS software may be affected by a bug in the PGS software (whch according to Peter Yantzer is not a problem with their more robust AGSL version).

So this and other debate that raged here for a while may have been a waste of time.

I would ask those with the PGS software to check the grade of a 65% table
Pavilion: 41.4 Crown: 33 Lower girdle facet length: 80% or Lower girdle facet depth: 81.5% and Stars of : 55% and 49%


all with a Girdle bezel of 3%
I have attached the 55% star .dmc

re:So this and other debate that raged here for a while may have been a waste of time.

I am disagree. Such debats are very helpfull for understanding what is AGS cut grade system is.

re:whch according to Peter Yantzer is not a problem with their more robust AGSL version).

I do not see reason to have two types source code: One for AGSL, other for sells PGS software.
At least it is not honest for PGS clients. Idea sells PGS software was to give possibility to clients do PRE-grading.
If PGS software do wrong pregarding, What is reason BUY PGS software? How client can use PGS software( just for fun?)

I think AGS are changing PGS software and adding new and new rules. ( And real AGS light metrics has less and less value for grading)

 

[Serg]

Btw. Peter Y. Did not inform about RIGHT grade from very ROBUST AGSL version software for stone

65% table Pavilion: 41.4 Crown: 33 Lower girdle facet length: 80% or Lower girdle facet depth: 81.5% and Stars of : 55% and 49%

Please think what this fact is mean.

 

[strmrdr]

Date: 3/31/2007 2:06:50 PM
Author: Serg
Btw. Peter Y. Did not inform about RIGHT grade from very ROBUST AGSL version software for stone

65% table Pavilion: 41.4 Crown: 33 Lower girdle facet length: 80% or Lower girdle facet depth: 81.5% and Stars of : 55% and 49%

Please think what this fact is mean.

What grade did the pgs software give it?
Im guessing by all the fuss ags0 or ags1...
looking over the stone in DC I see no reason it couldnt get that grade.

 

[strmrdr]

remember that any "proportion set" that passes the tests gets the grade.....
looking over that ccombo DC scores it in the same class as a lot of ags0 stones.
Which is why I still think charts are useless.

 

[adamasgem]

Date: 3/31/2007 2:06:50 PM
Author: Serg
Btw. Peter Y. Did not inform about RIGHT grade from very ROBUST AGSL version software for stone

65% table Pavilion: 41.4 Crown: 33 Lower girdle facet length: 80% or Lower girdle facet depth: 81.5% and Stars of : 55% and 49%

Please think what this fact is mean.

Seems to me that the stone, as described above is not very well defined, so how can anyone rin it. It also seems to me that you were going to bbe given a copy of the PGS as a professional courtesy, so you could play to your hearts content and generate your own "charts", instead of continously complaining, not that charts based on symmetric stones are of anymore use than FARCEWARE, or ANY other symmetric rendering of performanc, which has been covered ad nausium here.

I imagine that AGS software, just like DiamondCalc, is contiously evolving, and the inhouse AGSL version has more bells and whistles than the PGS commercial version, but also reqiures dual core processors for speed and has a lot of research material built in, not part of the commercial version, just like what you showed in April with DiamondCalc.

You harped on "problems with PGS, which were directly related to ascii to binary STL conversion problems with DiamondCalc, which are probably unavoidable, given the numerics, but it appears that your primary focus is to help in rough planning, and my hat is off to you in what you appear to done, but remember, SYMMETRY is a critical factor in the end grade, and unless you do the gradients and sensitivity analysis, it is sort of a shot in the dark.

Turn on the Russian equivilant of a Cray II or massive parallel processor, and let it grind away to see the localized maxima and minimas based on cutting assymetries for all the possible pertubations.

Hope to see you in Vegas, and keep up your good work..

 

[Serg]

Date: 3/31/2007 4:26:36 PM
Author: strmrdr

Date: 3/31/2007 2:06:50 PM
Author: Serg
Btw. Peter Y. Did not inform about RIGHT grade from very ROBUST AGSL version software for stone

65% table Pavilion: 41.4 Crown: 33 Lower girdle facet length: 80% or Lower girdle facet depth: 81.5% and Stars of : 55% and 49%

Please think what this fact is mean.

What grade did the pgs software give it?
Im guessing by all the fuss ags0 or ags1...
looking over the stone in DC I see no reason it couldnt get that grade.

AGS0( PGS result)

See DC result for Dark zone( 0.89), Fish eys(0.75), LR mono table(0.90)(. Its are too Bad for "Ideal" cut.
But it is not main point. The main point is Peter Y. answer: AGSL use diffirent software( method).
In such case what is reason to buy and use PGS?
It is main point.

 

[strmrdr]

Date: 3/31/2007 4:40:18 PM
Author: Serg

AGS0( PGS result)

See DC result for Dark zone( 0.89), Fish eys(0.75), LR mono table(0.90)(. Its are too Bad for ''Ideal'' cut.
But it is not main point. The main point is Peter Y. answer: AGSL use diffirent software( method).
In such case what is reason to buy and use PGS?
It is main point.

Thats not the numbers Im getting from Garry''s dmc file...
Im getting 97,97,89,1.0
Did Peter actualy check it or was that an off the top of his head answer?
per the AGS0 princess cut grading green counts towards contrast does the dark zone mono take this into account also?

 

[Rhino]

Sergey I hear you and agree.

Forgive me for not reading through this entire thread as I only caught the last 2 pages and had a moment to run the specs through the PGS.

Here are the results for 55% stars, all other parameters the same. Assuming this diamond had ideal polish/symmetry would get an AGS Ideal according to our PGS.

 

[Rhino]

Here''s the ASET generated from that file within the PGS.

 

[Rhino]

Here are the results of the identical measurements except changing the stars to 49% as Garry suggested. Interesting especially when you look at the accompanying ASET image which I''ll put in the next post.

 

[Rhino]

Accompanying ASET with 49% stars also generated within the PGS. All greens around the upper half region eliminated yet a 1 in light performance.

Good example Garry.

 

[strmrdr]

Date: 3/31/2007 5:56:33 PM
Author: Rhino
Accompanying ASET with 49% stars also generated within the PGS. All greens around the upper half region eliminated yet a 1 in light performance.

Good example Garry.

Greens count as contrast as shown by the ags princess cuts.
Thanks for proving it for me :}

 

[michaelgem]

The question the AGS may be asking themselves about now is whether this diamond with a 65% table is worth defending as Ideal.

Let me make the case that AGS should consider removing it and others with tables noticeably greater than 60% from the AGS Ideal 0.

The trade perception is that table sizes above 63% and certainly above 65% have a crown with a flat, spread appearance that is not perceived by cutters, jewelers or consumers as Ideal.

The GIA Excellent cuts off its allowed table size at 62% partially because of this perception.

In addition, looking at the enclosed DiamCalc image, it appears that the table reflection area has become too prominent, and is negatively affecting the diamond’s optical performance. The typical Ideal table reflection is nearer one-third the table diameter.

In my article, “The eye of the diamond”, ( http://www.acagemlab.com/articles/DiamondEye.htm ) I say: “When a diamond's pavilion is ideal cut, its pupil, or innermost ring of reflections will be about a third the size of the table. If this section is larger than one half the table, you are not looking at an Ideal cut diamond. Next, look at the iris, or middle, ring. If this table area appears noticeably greater than six-tenths of the diamond's diameter, you are also not looking at an Ideal cut diamond.”

Like Strm, my standards are even “tighter” and more conservative than this statement.

This diamond’s table reflection is just over half the table diameter, and the table size is noticeably greater than six-tenths of the diamond’s diameter.

I don’t know of anyone in the trade who would consider diamonds with table sizes above 63% as Ideal. I don’t see this diamond with 65% table as Ideal with respect to either historical craftsmanship standards or optical performance and beauty. It also does not make Ideal with respect to AGS’s and GIA’s original standards from the Liddicoat era.

Michael Cowing

 

[strmrdr]

Michael,
Good point, but the question becomes is this a matter of what we are used to thinking or is it a real downside?
After all ags ideal princess cuts have huge crowns and flat tops compared to what would be considered ideal in a round.
There are some cuts in AGS ideal that Id consider special use cuts rather than all around performers so my question is can this be one of them?
I don't know myself, the large tabled rounds iv seen don't have aset images like that.
interesting question and a much better one than the other.
What do you think?

edit too add: I wouldnt buy such a stone for a ring thats for sure, We agree there but I wonder if there is a use for a large tabled high performance round? (putting the above question another way)

 

[strmrdr]

here is the gem file for those that dont have DC and want to play.

 

[adamasgem]

Date: 3/31/2007 4:40:18 PM
Author: Serg

Date: 3/31/2007 4:26:36 PM
Author: strmrdr

Date: 3/31/2007 2:06:50 PM
Author: Serg
Btw. Peter Y. Did not inform about RIGHT grade from very ROBUST AGSL version software for stone

65% table Pavilion: 41.4 Crown: 33 Lower girdle facet length: 80% or Lower girdle facet depth: 81.5% and Stars of : 55% and 49%

Please think what this fact is mean.

What grade did the pgs software give it?
Im guessing by all the fuss ags0 or ags1...
looking over the stone in DC I see no reason it couldnt get that grade.

AGS0( PGS result)

See DC result for Dark zone( 0.89), Fish eys(0.75), LR mono table(0.90)(. Its are too Bad for ''Ideal'' cut.
But it is not main point. The main point is Peter Y. answer: AGSL use diffirent software( method).
In such case what is reason to buy and use PGS?
It is main point.

I believe the answer to that question highlighted above is an emphatic NO, much to your consternation. The "software" may be different but the scoring is using the same methodology, to my knowledge.

One might ask the same question about any software regarding getting it, shurly yours and mine always will have fixes and improvements, just as I suppose the AGS system will evolve, to handle different data formats and the problems generated by such, the example in this thread regarding conversion of DiamondCalc files..Sarin had similar small facet probelms, not to mention OGI. I don''t see you or Michael yelling and screaming about FARCEWARE(TM).

I imagine that as methodologies mature, certain things will change. I don''t think the MSU study or HCA is cast in stone.

And it all depends on the envirionment(s) one wants to adopt. And that is the biggest question. When I asked for a specific definition of the MSU, I got evasive answers, just like with certain measurement devices.

I personally think that the AGS methodology will evolve, and show photoreal representations, and may consider more than 0 and 15 tilt in the future, as I think a weighted analysis of the photoreal will tighten things up, and more metrics may be in the works.

 

[Serg]

Rhino, Thanks for confirmation

 

[Garry H (Cut Nut)]

Date: 3/31/2007 11:36:47 PM
Author: michaelgem

The question the AGS may be asking themselves about now is whether this diamond with a 65% table is worth defending as Ideal.

Let me make the case that AGS should consider removing it and others with tables noticeably greater than 60% from the AGS Ideal 0.

The trade perception is that table sizes above 63% and certainly above 65% have a crown with a flat, spread appearance that is not perceived by cutters, jewelers or consumers as Ideal.

The GIA Excellent cuts off its allowed table size at 62% partially because of this perception.

In addition, looking at the enclosed DiamCalc image, it appears that the table reflection area has become too prominent, and is negatively affecting the diamond’s optical performance. The typical Ideal table reflection is nearer one-third the table diameter.

In my article, “The eye of the diamond”, ( http://www.acagemlab.com/articles/DiamondEye.htm ) I say: “When a diamond''s pavilion is ideal cut, its pupil, or innermost ring of reflections will be about a third the size of the table. If this section is larger than one half the table, you are not looking at an Ideal cut diamond. Next, look at the iris, or middle, ring. If this table area appears noticeably greater than six-tenths of the diamond''s diameter, you are also not looking at an Ideal cut diamond.”

Like Strm, my standards are even “tighter” and more conservative than this statement.

This diamond’s table reflection is just over half the table diameter, and the table size is noticeably greater than six-tenths of the diamond’s diameter.

I don’t know of anyone in the trade who would consider diamonds with table sizes above 63% as Ideal. I don’t see this diamond with 65% table as Ideal with respect to either historical craftsmanship standards or optical performance and beauty. It also does not make Ideal with respect to AGS’s and GIA’s original standards from the Liddicoat era.

Michael Cowing

Michael while I would not agree that this is a nice set of #''s (it gets HCA 5.3), I think that in some markets, especially South East Asia and parts of Europe, this is a stone that is considered favourably.

But more importantly, it highlights what Sergey has always said - that designing cut grade systems based on the best stones is the wrong approach – it should start from the worst – it is like all empirical research – the best results come from research that is not setting out to prove what everyone already believes.

 

[Paul-Antwerp]

Date: 3/31/2007 11:36:47 PM
Author: michaelgem

The question the AGS may be asking themselves about now is whether this diamond with a 65% table is worth defending as Ideal.

Let me make the case that AGS should consider removing it and others with tables noticeably greater than 60% from the AGS Ideal 0.

The trade perception is that table sizes above 63% and certainly above 65% have a crown with a flat, spread appearance that is not perceived by cutters, jewelers or consumers as Ideal.

The GIA Excellent cuts off its allowed table size at 62% partially because of this perception.

Michael Cowing

The ruling perception in the 15th century was that the Earth was flat.

I was thinking about a more elaborate answer, but I think this clearly demonstrates that the current trade perception should be disregarded. Being at the center of the world''s trade, and personally knowing many leading diamond cutters and diamond technicians, I am just more convinced about this.

Live long,

 

[Paul-Antwerp]

Date: 3/31/2007 9:01:11 PM
Author: strmrdr

Date: 3/31/2007 5:56:33 PM
Author: Rhino
Accompanying ASET with 49% stars also generated within the PGS. All greens around the upper half region eliminated yet a 1 in light performance.

Good example Garry.

Greens count as contrast as shown by the ags princess cuts.
Thanks for proving it for me :}

Do not forget that the score does not result from the top-view only, but also from various degrees of tilt. One cannot deduct the minute difference of 0.05 in contrast from just the top-ASET-view.

Live long,

 

[strmrdr]

Date: 4/1/2007 7:20:50 AM
Author: Paul-Antwerp

Date: 3/31/2007 9:01:11 PM
Author: strmrdr

Date: 3/31/2007 5:56:33 PM
Author: Rhino
Accompanying ASET with 49% stars also generated within the PGS. All greens around the upper half region eliminated yet a 1 in light performance.

Good example Garry.

Greens count as contrast as shown by the ags princess cuts.
Thanks for proving it for me :}

Do not forget that the score does not result from the top-view only, but also from various degrees of tilt. One cannot deduct the minute difference of 0.05 in contrast from just the top-ASET-view.

Live long,

I look at em in dc over a wide range of angles.. the biggest difference is in the amount of green.
Remember all angles are not weighed equally and I''m willing to bet 2 donuts that the face up is heavily weighted for the contrast score.

 

[michaelgem]

To many, the lable 'Ideal' is the loftiest accolade that a 57 facet standard round brilliant can obtain. It would be a shame to see that standard diluted by diamonds that cutters, jewelers, and consumers see as less desirable in comparison.

Let me appeal to AGS and everyone to consider the following when deciding whether diamonds with table sizes noticeably greater than 60% should be blessed by AGS as Ideal:

In my 2000 Brilliance Article (http://www.acagemlab.com/articles/Ideal.htm ) I said: “Observations of the effects of diamond proportions on brilliance by diamond cutters from Tolkowsky to Watermeyer, and observations by people in the diamond trade and consumers provide the ‘litmus test’ for conclusions drawn from computer modeling of brilliance in diamonds.”

I was referring to GIA’s WLR metric for brilliance, but this statement applies equally well to conclusions about which diamonds are Ideal based upon AGS’s metrics for brightness, contrast, dispersion, etc.

A round brilliant with a 65% table fails this ‘litmus test’.

In the article, “Describing diamond beauty - assessing the optical performance of a diamond” ( http://www.acagemlab.com/articles/Describing.htm ) I state what I believe is a fundamental truth: “The worth of a measure of diamond beauty depends upon how well it agrees with human judgment. --- Perception of beauty is everything. ---“

Although diamonds with 41.4, 33 main angles are perceived to have Ideal or Excellent optical performance or beauty with table sizes under 63%, the same standard round brilliants but with tables noticeably greater than 60% are not perceived my most to be as beautiful. And in this particular case I observe a significant diminishment in optical performance in the table area.

This diamond and others with table sizes noticeably greater than 60% do not appear to deserve the accolade: AGS Ideal 0.

Michael Cowing

 

[Paul-Antwerp]

Michael,

I think that it was a gentleman called Garry Holloway, who started demonstrating ten to fifteen years ago that one should not look at one proportion-measurement separately to judge the beauty of a diamond. This is not only true in pavillion and crown angle, but also in table size.

This thread is a long repetition of you giving half information, and then changing it in a follow-up-post, because somebody showed you that you were incomplete or plainly wrong.

In this example, you first talk about ''trade perception'' and when I react to that, it suddenly changes into ''consumer''s litmus-test''. What is this? Science or a popularity contest?

Can you please stop the demagogy, and try to be constructive?

Live long,

 

[Garry H (Cut Nut)]

Michael will you please read my post on the last part of page 6 where I highlighted these proportions for the first time.

You will note that Peter yantzer told me that this set of proportions does not qualify for AGS 0 and that the lab software apparently does not have the bug that the public software has and the lab gives this stone a different result.

Meanwhile, part of your post below is back on the original topic of this thread.
That you should refer to GIA''s observational study as a ''litmus test'' seems to me to indicate that you eiither did not read the Letters to the Editor of the Aust Gemmo, or you did not understand our case, or you did not agree with us?

So in any of those cases perhaps you would like to participate in the original discussion that this thread was meant to cover?

Should GIA withdraw their grading system pending a thorough (ideally independant, if I could use that term) review of the science that underpins it.

There are many parts of that exchange of letters that you hold strong opinions about.

Date: 4/1/2007 11:28:12 AM
Author: michaelgem
To many, the lable ''Ideal'' is the loftiest accolade that a 57 facet standard round brilliant can obtain. It would be a shame to see that standard diluted by diamonds that cutters, jewelers, and consumers see as less desirable in comparison.

Let me appeal to AGS and everyone to consider the following when deciding whether diamonds with table sizes noticeably greater than 60% should be blessed by AGS as Ideal:

In my 2000 Brilliance Article (http://www.acagemlab.com/articles/Ideal.htm ) I said: “Observations of the effects of diamond proportions on brilliance by diamond cutters from Tolkowsky to Watermeyer, and observations by people in the diamond trade and consumers provide the ‘litmus test’ for conclusions drawn from computer modeling of brilliance in diamonds.”

I was referring to GIA’s WLR metric for brilliance, but this statement applies equally well to conclusions about which diamonds are Ideal based upon AGS’s metrics for brightness, contrast, dispersion, etc.

A round brilliant with a 65% table fails this ‘litmus test’.

In the article, “Describing diamond beauty - assessing the optical performance of a diamond” ( http://www.acagemlab.com/articles/Describing.htm ) I state what I believe is a fundamental truth: “The worth of a measure of diamond beauty depends upon how well it agrees with human judgment. --- Perception of beauty is everything. ---“

Although diamonds with 41.4, 33 main angles are perceived to have Ideal or Excellent optical performance or beauty with table sizes under 63%, the same standard round brilliants but with tables noticeably greater than 60% are not perceived my most to be as beautiful. And in this particular case I observe a significant diminishment in optical performance in the table area.

This diamond and others with table sizes noticeably greater than 60% do not appear to deserve the accolade: AGS Ideal 0.

Michael Cowing

 

[michaelgem]

Paul,

It is beyond my understanding why you see my post as demagoguery. I guess you have a different idea of the meaning of the word. I am obliged to respond, because others may feel as you do.

There is nothing incomplete or plainly wrong about the post before last, and the last post elaborates on my reasoning. It does not change it in the least.

If anything is plainly wrong it is your attribution to Garry of the statement that one should not look at one proportion-measurement separately to judge the beauty of a diamond.

That was a commonly made statement back in the Gemkey days, before Diamondtalk and before PriceScope. It was used by several entities and especially GIA to point out that systems like the old AGS Ideal 0 were incorrect in specifying ranges of proportions independently, since this implies rectangular boundaries on the range of Ideal.

For example, in their first article, Fall 1998, GIA says: “We know that one cannot, and must not, assess the cut of a diamond by examining any one of these proportion parameters alone.”

The charts show the interrelationship between crown angle and pavilion angle and table size. In the GIA system there are no combinations of crown angle and pavilion angle that are seen as Excellent in the charts past a 62% table.

Far from demagoguery, I am making a constructive contribution to these discussions. I hope some will look beyond any mischaracterization to see the worth that should be obvious to AGS in what I have to say.

I am suggesting that AGS consider correcting what most in the trade will see as an incorrect aspect of today’s AGS Ideal 0 grading system. The concept of Ideal should not be diminished by the addition of proportion sets with table sizes noticeably greater than 60%.

I will lay you 10:1 odds that the large Ideal manufacturers, including companies like Keppie-Kiger, Lazare Kaplan, Katz, Eurostar, Overseas Diamond etc. will agree. They will continue cutting Ideal 0’s with 60% or less table sizes, because they perceive that these diamonds have superior optical performance and beauty to diamonds with larger tables. They will resent it when diamonds with greater than 60% tables are given the same Ideal 0 grade as their diamonds.

And it is not just consumer observation that is the litmus test or a popularity contest. The litmus test is observations of the effects of diamond proportions on optical performance or beauty by diamond cutters from Tolkowsky to Watermeyer, and observations by people including myself in the diamond trade as well as consumers.

If their observations disagree with anyone’s metric for aspects of diamond beauty like brightness, contrast, dispersion or scintillation then the metric needs another look. Is it better to have blind faith in the output of a metric, even one as excellent as AGS’s, or should you question the metric logic when it disagrees with expert and perhaps even not so expert observation?

This should not be taken or seen as criticism, but rather as constructive thoughts and suggestions that could lead to an even better standard for Ideal.

In a joint essay in the AGS Lab News, which I fondly remember, Peter Yantzer, Tom Tivol and I stated: "The diamond''s optical performance, as seen by the trained eye, and aided by performance assessment devices, will become the final quality control measure that will be used to refine the range of "Ideal" proportions."

Michael Cowing

 

[Garry H (Cut Nut)]

Date: 4/1/2007 5:16:47 PM
Author: michaelgem

Paul,

It is beyond my understanding why you see my post as demagoguery. I guess you have a different idea of the meaning of the word. I am obliged to respond, because others may feel as you do. I think the word is very clearly understood in all Latin based languauges Michael, and you sail very close to the wind in that respect.

There is nothing incomplete or plainly wrong about the post before last, and the last post elaborates on my reasoning. It does not change it in the least.

If anything is plainly wrong it is your attribution to Garry of the statement that one should not look at one proportion-measurement separately to judge the beauty of a diamond.

That was a commonly made statement back in the Gemkey days, before Diamondtalk and before PriceScope. It was used by several entities and especially GIA to point out that systems like the old AGS Ideal 0 were incorrect in specifying ranges of proportions independently, since this implies rectangular boundaries on the range of Ideal.

For example, in their first article, Fall 1998, GIA says: “We know that one cannot, and must not, assess the cut of a diamond by examining any one of these proportion parameters alone.”

Michael at that time the GIA did not fully understand the issue of an inverse connection between crown and pavilion angles. You do know this. If you need a reminder here is a coment in an edit in something that I was writting to the GIA Cut Study Team written by Dr Ilene Rienitz - here comment is in brackets and was dated Aug 21 1999.
"The patterns in Figure 11 in the Gems & Gemology paper by Hemphill et al. show a trend. The centre point of the oval ‘bull''s-eyes’ on the seven graphs for crown angles between 30.5° and 36.5° move from the top right to the bottom left. This trend confirms my simple system for buying, unseen, diamonds with the aid of Sarin reports. (Please note that this is Mr. Holloway’s system, not one proposed or necessarily supported by GIA. Ilene this concept does work and will be the basis of systems of the future)

The charts show the interrelationship between crown angle and pavilion angle and table size. In the GIA system there are no combinations of crown angle and pavilion angle that are seen as Excellent in the charts past a 62% table.

Far from demagoguery, I am making a constructive contribution to these discussions. I hope some will look beyond any mischaracterization to see the worth that should be obvious to AGS in what I have to say.

I am suggesting that AGS consider correcting what most in the trade will see as an incorrect aspect of today’s AGS Ideal 0 grading system. The concept of Ideal should not be diminished by the addition of proportion sets with table sizes noticeably greater than 60%.

I will lay you 10:1 odds that the large Ideal manufacturers, including companies like Keppie-Kiger, Lazare Kaplan, Katz, Eurostar, Overseas Diamond etc. will agree. They will continue cutting Ideal 0’s with 60% or less table sizes, because they perceive that these diamonds have superior optical performance and beauty to diamonds with larger tables. They will resent it when diamonds with greater than 60% tables are given the same Ideal 0 grade as their diamonds. That is hardly the basis for good science, as Paul has pointed out.

And it is not just consumer observation that is the litmus test or a popularity contest. The litmus test is observations of the effects of diamond proportions on optical performance or beauty by diamond cutters from Tolkowsky to Watermeyer, and observations by people including myself in the diamond trade as well as consumers.

If their observations disagree with anyone’s metric for aspects of diamond beauty like brightness, contrast, dispersion or scintillation then the metric needs another look. Is it better to have blind faith in the output of a metric, even one as excellent as AGS’s, or should you question the metric logic when it disagrees with expert and perhaps even not so expert observation?

This should not be taken or seen as criticism, but rather as constructive thoughts and suggestions that could lead to an even better standard for Ideal.

In a joint essay in the AGS Lab News, which I fondly remember, Peter Yantzer, Tom Tivol and I stated: ''The diamond''s optical performance, as seen by the trained eye, and aided by performance assessment devices, will become the final quality control measure that will be used to refine the range of ''Ideal'' proportions.'' The word that was left out was round - how can the same word be applied to other shapes when by the same measure - they are less than ideal?

It is nice to hear and read your opinions Michael, but they are just that - your opinions. If you could find a way to make them a little less ivory tower authoritative, then we could discuss ideas with you, rather than be drawn into time wasting debate.

Michael Cowing

 

[michaelgem]

Date: 4/1/2007 12:11:34 AM
Author: strmrdr
Michael,
Good point, but the question becomes is this a matter of what we are used to thinking or is it a real downside?
After all ags ideal princess cuts have huge crowns and flat tops compared to what would be considered ideal in a round.
There are some cuts in AGS ideal that Id consider special use cuts rather than all around performers so my question is can this be one of them?
I don't know myself, the large tabled rounds iv seen don't have aset images like that.
interesting question and a much better one than the other.
What do you think?

edit too add: I wouldnt buy such a stone for a ring thats for sure, We agree there but I wonder if there is a use for a large tabled high performance round? (putting the above question another way)

Strmrdr,

I second all your thinking here.

To answer your question, I do think this is a matter of a real downside. Isn't the diminishment in optical performance apparant when you look at this image in diamcalc's disco lighting?

Everything that I have spoken of is in reference to what facetors call the Standard Round Brilliant (SRB) with 57 important facets. As Gabi Tolkowsky is fond of saying: This is just one beauty. There are many beauties. The finest form of this beauty is what has historically been called Ideal.

We can discuss where the point is, but at some point beyond a 60% table the SRB ceases to be an Ideal standard round brilliant. All the cutters that I have personally spoken to, including Americans, Belgians and South Africans, indicate that the point where they observe a diminishment in round brilliant beauty is at table sizes above about 60%. This is true even when you optimize the other parameters for the larger table size.

As I said earlier, SRB's with large tables and resulting thin crowns have a flat, spread appearance that is not perceived by cutters, jewelers or consumers as Ideal. In fact, the trade refers to these diamonds as 'swindled', which can hardly be construed to be Ideal.


At some point you no longer have an Ideal standard round brilliant, but what you refer to as a special use cut. I suggest that the special use is greater yield from the rough just as it is with the princess cut.

A spready table dominates the resulting thin crown squeezing out and reducing the positive impact of the other crown facets on the diamond's optical performance. As in the princess cut, this dictates changes in the pavilion faceting to compensate for this loss.

But now you have a different beauty, not the Ideal Cut. If cutters want to increase yield by cutting thin crown spready diamonds, this example is one good way to do it. But do not ask to have it blessed as something that it clearly isn't.

Michael Cowing

PS If you are going this route, I suggest longer pavilion halves, and you may as well forget about the large flash brilliance and fire that I call "the hallmark of the early Ideals from the time of Morse and Tolkowsky."

Michael Cowing

 

[Serg]

Big part table in RD with table +60(65) try collect light sources in direction +45 degree . Usually light is on central part ceiling and in central part wall. Light sources in corner between ceiling and wall are to rare.

real light distribution is much more complex then hemisphere with square cosines or Blue-Red-Green ASET.