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Another GIA press release on diamond cut research

Discussion in 'RockyTalky' started by mike04456, Feb 13, 2004.

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  1. mike04456
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  2. Garry H (Cut Nut)
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    by Garry H (Cut Nut) » Feb 13, 2004
    Thanks LM.

    Yes this is the issue we have discussed at great length.
    Girdle Cheating is the bad guy and the good guy is the 8* ACA newline method.

    We mention it here at the bottom of the page http://www.cutstudy.com/cut/english/symmetry/6.htm

    We have known since 1999 about the 8* process.

    The negative possability concept was discovered here on this forum by Sergey after a consumer posted a diamond's spec which did not add up to the correct weight. I think it would be fair to say that these forums have played a role in the science of cut grading.
     
  3. Garry H (Cut Nut)
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  4. Serg
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    by Serg » Feb 14, 2004
    Maybe this article was written in a hurry because at least two evident logic errors can be easily seen there. Really we see three but not sure that the third error is so rude.

    Maybe authors of this article have had no time to understand the main reasons why diamond manufacturers change an index (asimuth) of upper girdle facets.

    It is funny that the authors use such a phrase as "the stone on the left in figure B demonstrates, some approaches to cutting the upper and lower girdle facets produce a less pleasing appearance from essentially the same proportions." because the diamond under this description seems to be cut with the same approach like Eightstar diamonds.

    There is also interesting that they have written: "There is yet another important variable in the description of the round brilliant that we have not seen mentioned to date in the diamond literature". Maybe they do not read the diamond literature, and in particular an article in the magasine that they recieve from us maybe a half of year ago according to the magazine exchange program. This is an issue of the Gemological Bulletin No 2(9), 2003 with article about diamond symmetry published in both English and Russian and reproduced at our webpage (link to the article)

    S&Y
     
  5. elmo
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    by elmo » Feb 14, 2004
    The GIA is putting the final polish on their cut grading system and just figuring this out at the same time?

    Serg and Garry, dumb question: is changing the index of the girdle facets as described in the article the same thing as changing their azimuth, or are there two separate (but related) things going on? My uneducated guess was separate but related.
     
  6. Serg
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    by Serg » Feb 14, 2004

    Yes, index and azimuth describe the same coordinate of a facet.
     
  7. elmo
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    by elmo » Feb 14, 2004
    Ok, that makes sense...changing its index position effectively turns the facet since the index is a position on a circle (girdle). Thanks [​IMG].
     
  8. Rhino
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    by Rhino » Feb 14, 2004
    This comes at an interesting time. We just had a custom report made for our DiaVision analysis that we are making available to the public which now shows

    min/max/avg. upper girdle angles
    min/max/avg. upper girdle/star ratios
    min/max/avg. lower girdle angles
    min/max/avg. lower girdle length

    The scan from Sarin and imported into DiamCalc demonstrates the issues GIA is bringing up in this article.

    You can download the file free on our page on the minor facets.

    Serg, you may want to consider adding the light view from this GIA article into the GemAdvisor program so people can see this phenomena as an option in the program. I think that would be a good idea and it's already in the DiamCalc. Good idea?

    Peace,
     
  9. Serg
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    by Serg » Feb 14, 2004

    Hi, Rhino

    Diamcalc has light model "ISO" + observer Head. I think it is like GIA Light, but the angle of Head on Photos by Al Gilbertson is much bigger. I have not information about this angle. I think it is near 30 degree.
    P/s
    I like your site. Well done.
     
  10. Rhino
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    by Rhino » Feb 14, 2004
    Thanks Serg. Have you tried downloading the GoodOldGold.xml file and opening a DiaVision with it yet?

    I'll play around with ISO and user head in DC and see what the closest is to GIA pics.

    Regards,
     
  11. Garry H (Cut Nut)
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    by Garry H (Cut Nut) » Feb 14, 2004
    Nice work Rhino.

    Just read the GIA article properly.
    Wow!
    Have theybeen hiding on anothe planet?

    DrIR was a participaant in GemKey when Pete Yantzer posted a wire froam showing the girdle treatment of an 8* back in 1999. I still have the printed copy and confirmed it next time I saw an 8* in person.
    Pretty well all the participants knew straight away that this was the 8* trick and kept hush hush about it out of respect. this is the stone that is thinner at the mains and has a lower upper girdle angle - like the new line ACA - and
    Brian has always said he thinks he prefers the normal stone.

    It was not until a year or 2 ago that Sergey noticed an AGS 0 stone that had a 'cheated girdle' that someone made the public connection and spilled the beans on the 8* trick.

    Now if you have a good read of the GIA article (which BTW reffers to table 1 which is absent) that you realise that they have little idea of which does what, and they actually seem to recomend the girdle cheated stones that devious cutters are cutting to get good girdle thickness grades and increase weight retention.

    Now can someone tell me I have missed something?
    Rhino - you have a good handle on this now that many of your stones have indexed girdles.
    Sergey? Care to comment?
    Anyone else?
     
  12. Serg
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    by Serg » Feb 14, 2004

    Table 1.
    The three diamonds shown in figure B have very similar proportions.

    Proportion
    Commercial cut
    Hearts On Fire
    Fabrikant Brilliant

    Crown angle
    34.5°
    34.3°
    34.0°

    Pavilion angle
    40.9°
    40.6°
    41.1°

    Table percentage
    56%
    54%
    56%

    Girdle thickness
    4.3%
    4.0%
    3.0%

    Total depth
    62.5%
    61.6%
    62.3%

    Star length
    60%
    60%
    55%

    Lower girdle length
    80%
    80%
    80%

    Average amount of indexing
    3.5°

    – 4.0°


    link to table1

    Garry, now you can see second big mistake IR.
     
  13. Serg
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    by Serg » Feb 14, 2004
    Garry,
    About mass:

    Spread all this GIA examples is close.

    Deviation of index halves is not important for mass.
     
  14. Garry H (Cut Nut)
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    by Garry H (Cut Nut) » Feb 14, 2004
    thanks Sergey (silly me)

    It seems that a fair amount of the findings are dependant on the proportions rather than any other features.

    The lower part of this image has DiamCalc models to the proportions on table 1 with a similar lighting - the head shadow is about 39 degrees.

    Comparison for proportions only.jpg
     
  15. Serg
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  16. Serg
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    by Serg » Feb 16, 2004
    re: "Figure B shows three round brilliants with very similar proportions (table 1). They were photographed in a viewing environment that uses both diffuse white light and a black spot, which emphasizes the face-up contrast pattern of a diamond. Despite the similarity in proportions of the three diamonds, they display markedly different face-up patterns. These differences are due to the brilliandeering, or indexing, which is the only parameter that varies substantially among the three." This report was prepared by Ilene Reinitz and Tom Moses of the GIA Gem Laboratory in New York.


    ---------

    Garry, here is my attempt.

    I used GIA parameters from table 1, with the exception of index. Index is 0 for all 3 diamonds. Intensities of light are different for different diamonds.

    GIAPhotoDCImageArtFeb13_2004.jpg
     
  17. Garry H (Cut Nut)
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    by Garry H (Cut Nut) » Feb 16, 2004
    1. Sergey I would like to know (private or publicly) how you modelled your images?
    But certainly you would never have chosen these 3 stones as having "similar" proportions.

    2. I would also like to know what is going on in the minds of you GIA cut researchers?
     
  18. Iiro
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    by Iiro » Feb 17, 2004
    In this thread we are not only discussing about few pictures, but putting GIA's reputation and forthcoming Cut Grades under scope.
    If we here just point the errors of the GIA study, the mischief to the diamond industry may be bigger than the benefit. Now we are forming a camp against GIA, I doubt if it is necessary.

    Ofcourse, the rude mistakes of GIA must be adressed and corrected. Without critic GIA could easily mislead ordinary jewelers like myself. Serg has now pointed a fundamental error in GIA study. How will GIA respond?

    Is there any way to start discuss with GIA here? How to break their silency?
    But, its is much up to GIA if they choose to respond or not.
     
  19. Serg
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    by Serg » Feb 17, 2004
    Hi Sibelius,

    Good questions, Very good.
    I think it is important topic for discussions on IDCC.
    This camp is necessary for work only, not for any war.
    GIA can work in this camp too.
    p/s When do you plan send abstract? We have a very good news about IDCC. [​IMG]
    pp/s You can find other big( But less important for Cutstudy) mistake in GIA article. Please try do it himself.
     
  20. Stephan
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    by Stephan » Feb 18, 2004
    /idealbb/files/faces_04.gif Am I right by thinking that:


    • 8star changes the index of the lower girdle?
    • ACA (New Line) changes the index of the upper girdle?

    Thank you, experts!

     
  21. Serg
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    by Serg » Feb 19, 2004

    Below is answer from Garry Holloway(He can not log in ):

    "8* change the upper girdle mostly and ACA do the same thing. I am not aware that indexing the pavilion has such a great effect


    Kind Regards

    Garry Holloway"
     
  22. Iiro
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    by Iiro » Feb 19, 2004
    Hi Serg,

    I have sent the abstract and will start reading again GIA study.
     
  23. Stephan
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    by Stephan » Feb 19, 2004


    Thanks Serg and Garry!




    Here is a girdle of a SuperbCert Diamond.


    SuperbCert doesn't change any index, right?




    /idealbb/files/superbcertgirdle.jpg



    source: http://www.superbcert.com



    After browsing GoodOldGold, I found some EightStar Diamonds.


    Some of them have a Megascope-report, and all the girdles look this way:


    /idealbb/files/8stargirdle.jpg



    source: http://www.goodoldgold.com



    My mistake in the post above is that I didn't notice that the crown side of the girdle was at the bottom of the picture on Megascope-reports.


    Sorry, I am sometimes so stupid!!!


    [​IMG]
     
  24. Iiro
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    by Iiro » Feb 22, 2004
    Serg,
    I am not sure of how many errors there are in GIA study, most of them are mentioned already.

    But you said I should find something myself.

    Here is my attempt.

    1) They fail to recognise earlier studies of the same subject. Particularly they not mention MSU study, witch was sent to them half a year ago.

    2) They try explain differencies in the looks of diamonds by different indexing, but the real reason is different proportions. ( Sergs picture of three dimonds)

    3) They are dead wrong about the effect of indexing and weight retention. In figure A the left stone has a much greater girdle thickness compared to right stone. If the girdle thickness were same, the right stone would be hevier.

    So far these are obvious mistakes, is there more big ones lying under?
     
  25. Stephan
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    by Stephan » Feb 22, 2004



    Sibelius, I think you are wrong about the girdle.


    Look at the red circles, the girdle at the bezel is the same for the 3 stones.




    /idealbb/files/samegirdle.jpg
     
  26. Iiro
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    by Iiro » Feb 22, 2004
    If someone cuts a stone like the left one, he makes thinner girdle. The outcome is lighter.

    If someone cuts a stone like the right one, he can hide thick girdle. The outcome is cheated to be heavier despite "light" looks.
     
  27. Stephan
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    by Stephan » Feb 22, 2004


    Oh, I understand what you mean!


    I am really stupid.
     
  28. Iiro
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    by Iiro » Feb 22, 2004
    Hi Stephan,
    Dont say you are stupid [​IMG]

    Just say your logic follows the GIA thinking [​IMG]
     
  29. Garry H (Cut Nut)
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    by Garry H (Cut Nut) » Feb 23, 2004
    Sibelius is right - the USA labs all measure the girdle at the thin part.
     
  30. Iiro
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    by Iiro » Feb 24, 2004
    There is also another thing.

    I do not know why the sideview of two of the three diamonds are drawn without one two pairs of girdle facets.
    [​IMG]
    In this picture there is 8 lower and upper girdle facets, the diamond is drawn without indexing of minors.

    Why are these missing in two GIA pictures? The third has little ones, but too little.
     
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