Consumer advisory: GIA Cut Grade Rounding Problems

[Serg]

Date: 2/5/2006 10:49:57 PM
Author: adamasgem

Date: 2/5/2006 9:44:13 PM
Author: strmrdr
34.2, 41, 56t,60%star,80% lgf
Marty''s lighting 0 to 15 degrees tilt.

Please understand that I do not know the distribution characteristics of the DC COS lighting scheme.... Anyone know Sergey? I just pulled it up on Sergeys suggestion as a non uniform distribution, so I don''t necessarily ''bless'' it, so to speak..

I reallly would like to see an altered Moon and Spencer Cloud covered sky model and/or a Pokrowski blue sky model used..

re:Anyone know Sergey?

(Uniform hemisphere)*COS(angle from Zenith) - Head

re:I''m thinking about two realistic lighting models based on the distributions I published before. Lower angle lighting is attenuated in amplitude and color by the local envirionment, trees, buildings, etc

Yes. Classical standards are not good for city. We will add HDR texture.

 

[Serg]

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[Garry H (Cut Nut)]

The aGS on-line grade based on the Helium 3D scan.

Note the stone is only downgraded to AGS 1 for light performance. i trust that the AGS system would be smart enough to downgrade it to 3 or 4 if the stone size was 2ct - because as it got larger the lack of contrast and scintillation would be more of an isssue. but in 10 points this would be a desirable stone.

To call this stone Poor is just plain Stoopid.


However the stone is downgrade because the girdle is over 6% at the minor facet junctions - and this time gIA wins. They describe the stone as Medium to slightly thick - which is a fair representation.
AGS should not be downgrading the stone for its girdle because the spread has not been affected - it is still a better spread than a Tolkowsky of the same weight.

It might be a little more difficult for a setter to handle - but not a real problem.

Of course Bill will not like it from a crafstmanship point of view - but as an aspiring Economist I think it shows enormous creativity - to take a piece of rough and push it to 1/2ct price point - without producing a bad stone - very creative - and requiring far more skill than is meant to exist in India.

 

[Garry H (Cut Nut)]

Check out this for craftsmanship.
Note how consistant the girdle is all around - and how the bruter has kept the square outline of the original 4 point rough (sawn across the table to give a pre bruted outline of a 4 pointed square).

All these things were required to get a magic 1/2ct weight.

Some of you think there is greater skill in producing a text book Tolkowsky - but you are wrong. This is crafsmanship on the edge of perfection

 

[strmrdr]

youch poor thats even harsher than me.
There are a lot worse performing stones out there that deserve the poor title more than this one.
ags 1 light performance is a gift in my opinion.
The performance on this one would be pretty one sided.
I like bright stones and would consider this one for a pendant but for a ring?
Id want more fire and scint.
To be honest on this one I disagree with both labs on the proper grade.
At best its a good to v-good cut in my opinion with being more towards the good side for a ring.
The lack of leakage is why id consider it for a v-g grade.

 

[Serg]

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I mean weight-girdle penalty only.

 

[Serg]

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[Serg]

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[adamasgem]

Date: 2/6/2006 12:02:59 PM
Author: Serg
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Sergey or Garry..

Would you please explain the PowerEtas plots above. I believe I know what is going on (chromatic flare), but I'm not sure exactly what you are doing here-- returned energy in the hemisphere north pole view ???
I'm sure the other readers might also be confused..

 

[adamasgem]

Date: 2/6/2006 5:47:22 AM
Author: Garry H (Cut Nut)
To call this stone Poor is just plain Stoopid.

Well put Garry...

 

[Serg]

Date: 2/6/2006 12:13:27 PM
Author: adamasgem

Date: 2/6/2006 12:02:59 PM
Author: Serg
.

Sergey or Garry..

Would you please explain the PowerEtas plots above. I believe I know what is going on (chromatic flare), but I''m not sure exactly what you are doing here-- returned energy in the hemisphere north pole view ???
I''m sure the other readers might also be confused..

Marty,

Short Explanation from different sides,
Like Light return for Hemisphere light condition weighting By cos*cos. But is non correct explanation. Because it is ability to see light source 5 mm ( distance from light to diamond is 2000mm, distance from viewer to diamond is 600 mm) by human eye with purple 4 mm.
Its is not easy explain shortly , I need a lot of drawings and time .

Firstly please see better explanation http://www.gemology.ru/cut/english/conference_posters/4.htm

+
ETAS - black/white
Cetas-colorEtas
Netas -negative Etas

 

[He Scores]

Date: 2/6/2006 6:01:05 AM
Author: Garry H (Cut Nut)
Check out this for craftsmanship.
Note how consistant the girdle is all around - and how the bruter has kept the square outline of the original 4 point rough (sawn across the table to give a pre bruted outline of a 4 pointed square).

All these things were required to get a magic 1/2ct weight.

Some of you think there is greater skill in producing a text book Tolkowsky - but you are wrong. This is crafsmanship on the edge of perfection

Gary,
There are many skills involved with cutting diamonds. Accurate girdling like your example here is one skill and often done by one person who just girdled the stone. Another person had to carefully block it, know to leave the top mains on a flatter angle leaving the thicker girdle and the brilliandeerer may have save another half point or point or so by "pasting halves" on or could have lost the half carat weight by digging them in a dramatic fashion. The blocker also had to "leave" the bottom angles high to save the weight. Had he put the bottom angles on 40.75, invariably the .50 weight would have lost. In addition to these ''elements'', there are a great many more that could be looked at to determine the cutter''s skills other than just the even girdle (and I suppose remaining naturals, which I can''t really determine from the data). If that is the case regarding the naturals...top top girdling workmanship would be to have all four or two naturals exactly the same size.
To look at this stone''s cert and accept "averages" of the main angles and say that it''s on the edge of perfection stretching it, I believe. Additionally, the bottom halves seem to on the high side...this would deminish the definition between the bottom halves and bottom mains.

Would this stone sell at a higher price than a "premium" cut in the 40''s? Perhaps, perhaps not. But regardless of the cutter''s skills in saving the weight, few people would regard this stone as a top cut. The question for the cutter is " cut for make or cut for weight". One takes certain skills, the other takes other skills. For the most part, the skill of the cutters may be equal except that the priority to where those skills are used is different.

So it''s not wrong for those of us too consider an accurate Tolkowsky model as a bench mark for workmanship, rather it''s wrong to take one aspect of cutting done well and label it as the edge of perfection for cutting in general.



Bill

 

[adamasgem]

Date: 2/6/2006 12:51:14 PM
Author: Serg

Date: 2/6/2006 12:13:27 PM
Author: adamasgem

Date: 2/6/2006 12:02:59 PM
Author: Serg
.

Sergey or Garry..

Would you please explain the PowerEtas plots above. I believe I know what is going on (chromatic flare), but I''m not sure exactly what you are doing here-- returned energy in the hemisphere north pole view ???
I''m sure the other readers might also be confused..

Marty,

Short Explanation from different sides,
Like Light return for Hemisphere light condition weighting By cos*cos. But is non correct explanation. Because it is ability to see light source 5 mm ( distance from light to diamond is 2000mm, distance from viewer to diamond is 600 mm) by human eye with purple 4 mm.
Its is not easy explain shortly , I need a lot of drawings and time .

Firstly please see better explanation http://www.gemology.ru/cut/english/conference_posters/4.htm

+
ETAS - black/white
Cetas-colorEtas
Netas -negative Etas

Thanks Sergey
Would you post a Helium scanned stone dmc file for the example stone so that the DC reports would be filled in. They are not filled in with the originak gem file..

 

[Garry H (Cut Nut)]

Marty i will email you and a few others a full DiamCalc.

tell me if you would like a copy garryh @ ideal-scope.com
If you open the File area in the top left you will see complete report data.

Bill - I agree with everything you wrote
Except the stone's angular variance is actually quite small when measured thru the axias rather than from the table reference point.

 

[adamasgem]

Date: 2/6/2006 11:24:47 PM
Author: Garry H (Cut Nut)
Marty i will email you and a few others a full DiamCalc.

tell me if you would like a copy garryh @ ideal-scope.com
If you open the File area in the top left you will see complete report data.

Bill - I agree with everything you wrote
Except the stone''s angular variance is actually quite small when measured thru the axias rather than from the table reference point.

Thanks Garry for the dmc file. Garry couldn''t post it because of the file size (332kb)...

Has anyone tried to do a Sarin scan of the same stone?? That would be interesting to compare..

 

[Garry H (Cut Nut)]

Date: 2/6/2006 11:52:38 PM
Author: adamasgem
Thanks Garry for the dmc file. Garry couldn''t post it because of the file size (332kb)... Thasnks Marty - I should have mentioned that

Has anyone tried to do a Sarin scan of the same stone?? That would be interesting to compare..

Sergey pointed out even Ogi should do an OK job of the scan because - even though there is only a degree or less between the pavilion minors and mains - they show up clearly in profile view.
The true test of a scanner is the accuracy of azimuth - and here there is a more clearly defined azimuth than on a regular stone.

 

[valeria101]

Despite the warm recommendations, Garry, I am guessing that such diamond will have a hard time holding a good name against adverse ''priming'' - even w/o bad scores on a cert.

 

[Serg]

Date: 2/7/2006 12:17:51 AM
Author: Garry H (Cut Nut)

Date: 2/6/2006 11:52:38 PM
Author: adamasgem
Thanks Garry for the dmc file. Garry couldn''t post it because of the file size (332kb)... Thasnks Marty - I should have mentioned that

Has anyone tried to do a Sarin scan of the same stone?? That would be interesting to compare..

Sergey pointed out even Ogi should do an OK job of the scan because - even though there is only a degree or less between the pavilion minors and mains - they show up clearly in profile view.
The true test of a scanner is the accuracy of azimuth - and here there is a more clearly defined azimuth than on a regular stone.

re:Sergey pointed out even Ogi should do an OK job of the scan because - even though there is only a degree or less between the pavilion minors and mains - they show up clearly in profile view.

Garry,

I did not point it!

My point. Small slope difference between main facet and girdle pavilion facet( in round cut) is not problem for any scanner.

Small azimuth difference between such facets is problem for all currents scanners.

This diamond has VERY small azimuth difference( This diamond is difficult for all scanners. I do not think OGI or even Sarin can give good result for Pavilion)

P/S If You did not understand anything from private correspondence do not good idea for put it forum. Firstly understand it , please. Do not put your statements to my mouth, please.

 

[strmrdr]

My bottom line question to Gary, serg, Bill and Marty is what do you think is a fair grade for this stone and why?


edit: would it be possible to send this diamond to Jon or RockDoc for a run on the b-scope and to Dave for a imagem report?

 

[Serg]

Date: 2/7/2006 2:58:02 AM
Author: strmrdr
My bottom line question to Gary, serg, Bill and Marty is what do you think is a fair grade for this stone and why?


edit: would it be possible to send this diamond to Jon or RockDoc for a run on the b-scope and to Dave for a imagem report?

Strict (conservative) grade
Very Good+ for diamond less than 0.3 ct( because for small diamond Brightness is very important. )

Good+ for Diamond more than 1.5 ct.( Low contrast due combination Pavilion and crown angles, Not due painted. )

Good- or even Fair ( Such parameters WITHOUT PAINTED, PAINTED IS Strongly increasing performance( not Brightness only, I think such painted increase Fire and Scintillation too (for such parameters))

Non conservative grade

Excellent for diamond less than 0.3 ct( because for small diamond Brightness is much more important then Scintillation and Fire)

Very Good+ for Diamond more than 1.5 ct.( Low contrast due combination Pavilion and crown angles, Not due painted. )

Very Good- or even Good+ ( Such parameters WITHOUT PAINTED)

 

[adamasgem]

Date: 2/7/2006 2:58:02 AM
Author: strmrdr
My bottom line question to Gary, serg, Bill and Marty is what do you think is a fair grade for this stone and why?


edit: would it be possible to send this diamond to Jon or RockDoc for a run on the b-scope and to Dave for a imagem report?

For the 0.5ct stone as it is.. Good, primarily because of the girdle extra weight hitting the weight boundary. I wouldn''t discount it as much if it were a 0.55ct, I give it a Good +

 

[Garry H (Cut Nut)]

I would also think GIA should be Good and AGS should be 4-6 depending on their girdle rules (which a stone like this shows possible errors in). GIA Poor shows that their system has limited ''intellegence''.

If the girdle was thinner -say 4.9% at thickest part, then I would be happy with AGS 4 - and maybe then it would be closer to GIA border of Very Good.

AGS 1 for 1/2 ct is too high I think.
I agree with Storm that it would be a good pendant stone.

My grading does not down grade a stone for an overly thick girdle unless the spread has got smaller.

In this case the spread is very good (+1%).

It is a very interesting stone.

 

[Garry H (Cut Nut)]

Date: 2/7/2006 2:06:02 AM
Author: Serg

re:Sergey pointed out even Ogi should do an OK job of the scan because - even though there is only a degree or less between the pavilion minors and mains - they show up clearly in profile view.

Garry,

I did not point it!

My point. Small slope difference between main facet and girdle pavilion facet( in round cut) is not problem for any scanner.

Small azimuth difference between such facets is problem for all currents scanners.

This diamond has VERY small azimuth difference( This diamond is difficult for all scanners. I do not think OGI or even Sarin can give good result for Pavilion)

P/S If You did not understand anything from private correspondence do not good idea for put it forum. Firstly understand it , please. Do not put your statements to my mouth, please.

OK, i understand now Sergey.

Here is an example of the Helium Scan azimuth angles for this stone on the left, and a normal stone on the right.

What you are saying is scanners have a big problem measuring this azimuth from shadow edge information only. The difference between the mains and lower half factes is so very close.

 

[Serg]

re:edit: would it be possible to send this diamond to Jon or RockDoc for a run on the b-scope and to Dave for a imagem report?

I try buy this stone. Did not receive price yet.( May be I am not first in order

 

[valeria101]

Date: 2/7/2006 6:01:09 AM
Author: Garry H (Cut Nut)


My grading does not down grade a stone for an overly thick girdle unless the spread has got smaller.

Why do the labs do this? Is there something technically wrong with a thick girdle regardless?

AGS''s ''spread'' that is something else than size seems confusing

 

[Serg]

Date: 2/7/2006 6:06:48 AM
Author: valeria101

Date: 2/7/2006 6:01:09 AM
Author: Garry H (Cut Nut)


My grading does not down grade a stone for an overly thick girdle unless the spread has got smaller.

Why do the labs do this? Is there something technically wrong with a thick girdle regardless?

AGS''s ''spread'' that is something else than size seems confusing

re:Why do the labs do this? Is there something technically wrong with a thick girdle regardless?


They use traditional approach for untraditional diamonds.

 

[Garry H (Cut Nut)]

My rules are that grdles that are too thin should be downgraded.

Diamonds with bigger spread should be upgraded (as in HCA - but this means downgrading girdles that lead to poor spread).

A diamond with a good spread because the crown is shallow should have at least a medium girdle (not Thin or Very Thin) to stop it chipping.

There is no such thing as a girdle that is TOO thick, unless it reudces beauty appearance or spread.

I prefer to grade for durability and spread and let cutters creatively find any way that makes a beautiful diamond.
This stone girdle does not reduce comparitive spread and it is not thick enough to appear bad (about 0.3mm at thickest part).

 

[Serg]

Date: 2/7/2006 6:41:55 AM
Author: Garry H (Cut Nut)
My rules are that grdles that are too thin should be downgraded.

Diamonds with bigger spread should be upgraded (as in HCA - but this means downgrading girdles that lead to poor spread).

A diamond with a good spread because the crown is shallow should have at least a medium girdle (not Thin or Very Thin) to stop it chipping.

There is no such thing as a girdle that is TOO thick, unless it reudces beauty appearance or spread.

I prefer to grade for durability and spread and let cutters creatively find any way that makes a beautiful diamond.
This stone girdle does not reduce comparitive spread and it is not thick enough to appear bad (about 0.3mm at thickest part).

re:I prefer to grade for durability and spread and let cutters creatively find any way that makes a beautiful diamond.


I am agree with all post.
I want add :
Labs need separate performance grade and spread grade. If thick girdle is bad for performance, they should downgraded. If thick girdle is not problem for performance let to consumer and seller to discuss about discount for total grade. Discount for 1.00 ct and 1.10ct with same negative spread should be different. Labs are doing outsider work badly, and destroy other good work( mix spread and performance is not good idea) Labs should help, clarify, But they misleading.

 

[adamasgem]

Date: 2/7/2006 6:06:48 AM
Author: valeria101

Date: 2/7/2006 6:01:09 AM
Author: Garry H (Cut Nut)


My grading does not down grade a stone for an overly thick girdle unless the spread has got smaller.

Why do the labs do this? Is there something technically wrong with a thick girdle regardless?

The extra weight in the Girdle plane does nothing to enhance beauty, can make setting a problem (so I''ve heard) and costs consumers money. That extra 0.01 carat may make it the stone cost 10-20% more..$$$$$$

 

[Serg]

Date: 2/7/2006 12:16:16 PM
Author: adamasgem

Date: 2/7/2006 6:06:48 AM
Author: valeria101

Date: 2/7/2006 6:01:09 AM
Author: Garry H (Cut Nut)


My grading does not down grade a stone for an overly thick girdle unless the spread has got smaller.

Why do the labs do this? Is there something technically wrong with a thick girdle regardless?

The extra weight in the Girdle plane does nothing to enhance beauty, can make setting a problem (so I''ve heard) and costs consumers money. That extra 0.01 carat may make it the stone cost 10-20% more..$$$$$$

Marty,
If painted increase performance , save girdle thickness in bezel place( Only one right place for definition girdle thickness) , increasing weight on 0.5-3%
should Labs downgrade such diamond on 1,2,3,4 grades?