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MRB diamond cut and things I don’t understand about it

Discussion in 'RockyTalky' started by kmoro, Apr 20, 2019.

  1. kmoro
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    by kmoro » Apr 20, 2019
    Happy Easter Weekend!

    Today I was doing the internet thing and found the following chart ... it made me feel better about my diamond’s 56.4% table (well, to be realistic, I stopped being concerned about it as soon as I saw the diamond is very fiery ... I still overthink things ...) ... sometimes was thinking about trying for that 55% table in the future (although not a giant leap from 56.4) ... thanks to PS ...and I believe that the smaller table produces more fire like everyone here says ... even just today learned a bit about the upper girdle facet relationship with the table size ... but what about this chart? Is it just another BS chart with a capital B and capital S that the experts here hate so much? I’m surprised that the smaller 53-55% tables are not included in the super-ideal range ... and that the depth is 61 - 62.5%.

    088FB01F-3A86-484F-878F-DFD81A5930AB.png
    https://www.petragems.com/education...cellent-ideal-cut-diamond-diamond-brilliance/

    I am also really doubting my ability to see the difference between a smaller table (say 54%) vs a larger table (say 60%) and the respective fire, brilliance, scintillation, produced by each.

    The Jann Paul videos refer to the contrast areas as the areas that produce fire (blue areas under the ASET scope) ... and it does appear that the colour flashes come off of these areas ... but from what I have read, it is still the crown facets that refract the light and “produce” the fire.

    I’m thinking that the fire produced by any individual diamond is the combination of the light refraction from the crown and how that split light reflects off of the pavilion mains. So ... the combination of the two - the angle relationship of the two - would determine how the coloured light is produced and reflected back to the eye, right? Would this explain why some 60/60 stones still have great fire while some 55% table diamonds don’t? And those coloured flashes off the pavilion mains would also be affected by the lower half lengths (shorter LH would be bigger flash vs skinnier flash), right? ETA: does the light refract again when coming OUT of the upper facets?

    I have noticed that 77% lower halfs seem to be the “safe spot” - what are the other factors that could make 70% or 85% undesireable? Is a large table with long lower half lengths bad or good, for example? Is it just personal preference or are some combos just automatically bad, as with crown angle and pavilion angle combos?

    I’m still trying to figure out the effect of star length on light performance other than how it affects the size of the other facets, like the upper girdle facets - maybe there is no other isolatable effect and these things must necessarily be taken as a whole in regard to diamond performance? Can anyone tell me more?

    I found this article on PS and taking the liberty of posting the link here ... for anyone new ... or anyone just wanting to look at an article about cut I guess. I liked this one for a lazy afternoon :twisted2:

    Laboratory cut grades and what the report doesn’t show - by John Pollard.
    https://www.pricescope.com/articles/laboratory_cut_grades_what_report_doesnt_show

    ETA#2 .... this picture chart from the above article ... I can see how the 60/60 stones do look whiter with less contrast ... quite a bit less contrast. Hmmm.

    AEFD2A7C-D8CA-4904-B0A8-6EE64CB7789B.jpeg

    Happy Saturday to you!
     
    Last edited: Apr 20, 2019
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  2. Karl_K
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    by Karl_K » Apr 20, 2019
    The chart and the reasoning behind it is garbage.
    60 table is easy to tell from 55 both in looks and performance.
    more later as its going to take a while and its family time.
     
  3. Johnbt
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    by Johnbt » Apr 20, 2019
    Hi. I just came inside after mowing a couple of acres. I clicked the first link and gave it a quick scan. First, I saw a lot about GIA. Then I saw a reference to the famous Mercel Tolkowsky. gah.gif

    I mean seriously folks, even I know his name is Marcel. Who writes this stuff?

    Okay, I'll get back to the rest of your post later, but now I'm off to grab a beer and a shower before supper.

    And a HAPPY EASTER to you too. I saw a baby rabbit while I was mowing and when I turned the corner behind the grape arbor one of the big ones sat there and stared me down. The riding mower didn't bother it at all.
     
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  4. kmoro
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    by kmoro » Apr 20, 2019
    Thanks! I think I answered some of my own questions with the picture chart that I added .... my biggest disadvantage is that I can’t compare diamonds in real life, and I have extremely - and I mean extremely - limited IRL diamond views ... and even fewer chances to compare diamonds IRL. Seeing a diamond without having others for comparison at the same time seems to make a really huge difference. I have a feeling that I would like any diamond IRL if in the right light and there was nothing to compare it to, lol (ok, not quite, lol, but you get my point).

    I think I should take a trip to Idaho :P2 ETA (of course): I would include Texas but it’s a lot farther away.
     
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  5. OoohShiny
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    by OoohShiny » Apr 20, 2019
    re: lower halves, I think GIA rounds massively!
     
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  6. OoohShiny
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    by OoohShiny » Apr 20, 2019
    This is the crux of the issue for all of newbies to diamonds - how can we see a range of cuts to understand and work out what we like, when local shops don't have the time, inclination, or in many cases, the actual knowledge to do so?

    I think all we can do is read, read, read on here, and hope that we understand the nuances and develop our 'palate' virtually, even if not experientially.
     
  7. chamois
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    by chamois » Apr 20, 2019
    I hope this isn’t a daft question.......but I have always wondered if there would be a visual difference (if all the other numbers are complimentary) in a table of 54 to 56?

    I too wish I could see and compare a slew of great performing diamonds with my own eyes.

    Thanks for posting this :wavey:
     
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  8. kmoro
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    by kmoro » Apr 20, 2019
    Me too, the question of 54 to 56!!
     
  9. Karl_K
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    by Karl_K » Apr 20, 2019
    1st off that a well cut modern Ideal cut has better fire than a well cut 60/60 style stone can be false.
    In lighting highly conductive to fire both are going to put on a light show and the 60/60 might just win.
    The difference in fire is that modern ideal cut potentially could show fire in more light conditions than the 60/60.
    That is where the 60/60 tends towards brightness comes from not that the 60/60 will always have inferior fire.
    As with anything with diamond performance it all depends on lighting!
     
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  10. Karl_K
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    by Karl_K » Apr 20, 2019
    Lowers:
    77-78 are near universal they will work very well with the widest range of angles/%/and table size.
    A steep pavilion can be countered by a very long lgf% to eliminate under table leakage but it will not fix light entrapment(facing up a darker color for the materials color.)
    https://www.pricescope.com/articles/do_pavilion_mains_drive_light_return_modern_round_brilliant

    70 is to short to really be a modern round brilliant and to long to really be considered an OEC type.
    Some people will call them transitional but in reality they were not really transitional.

    Basically there are different types of fire.
    Fire produced by the mains and fire from the rest of the diamond.
    Shorter lowers produce larger fire over the mains(they are bigger) but smaller fire other places.
    Longer lowers produce smaller fire over the mains(they are smaller) but larger other places.
    Like anything else its a balancing act.
     
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  11. Karl_K
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    by Karl_K » Apr 20, 2019
    The stars do have some effect on the area around and under them but the main thing they do is define the upper girdle size which then translates into upper girdles angles.
    Bigger stars can give a stone with a small table a more open look, but you have to watch the upper pavilion angle.
    But basically the steeper the crown angle and/or larger the table the more you have to look at the stars and upper girdle angles.

    So if you pavilion is steep and your running into issues with the upper girdle angles you can use smaller stars.
    If you have a shallow crown angles you can use longer stars and intentionally steepen the lower girdles up to a point and get some scintillation gain.
    Some of my favorite diamonds were 55t/34/41/80%/with 60%+ stars.
     
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  12. kmoro
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    by kmoro » Apr 20, 2019
    Thank you again, @Karl_K!!! The explanation is awesome and much appreciated!!!

    :wavey:
     
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  13. Karl_K
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    by Karl_K » Apr 20, 2019
    They would be very similar and very very hard to tell apart if they were not side by side.
    Even side by side it would not be that easy.
     
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  14. Dancing Fire
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    by Dancing Fire » Apr 21, 2019
    Everyone have their own fav specs. Am I wrong that my fav specs is a small table 54-55% on a high 15.5-15.8% crown? [​IMG]
     
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  15. Karl_K
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    by Karl_K » Apr 21, 2019
    Dont see a problem with it as long as the pavilion matches.
     
    


    


  16. Karl_K
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    by Karl_K » Apr 21, 2019
    Correction:
     
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  17. Karl_K
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    by Karl_K » Apr 21, 2019
    nearest five %
    What is ridiculous is the most universal 77-78 one is listed as 75 and the other 80.
     
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  18. OoohShiny
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    by OoohShiny » Apr 21, 2019
    I don't understand how GIA can justify their position on this, given how even small changes can make a large difference.

    I mean, why not have whole-number-only crown and pavilion angles?

    Why not have tables rounded to the nearest 5%?

    It makes no sense.
     
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  19. John Pollard
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    by John Pollard » Apr 21, 2019
    First, @Karl_K's lighting comments are fully applicable.

    Also, if you're in the camp that considers leakage a contrast component you see that increase for many of those examples against the black background. There's a photo in the linked article showing two diamonds against skin. One windows enough that you can see through a portion of the table to the skin underneath. Is that contrast? You be the judge.

    Since it's a holiday, let me toss another log onto the fire (pun intended)... The next person who buys a diamond and keeps it perfectly still, peering from 90 degrees perpendicular to the table only, will be the first person to do that.

    I'll just leave this here.
    https://www.pricescope.com/communit...ffect-face-up-performance.87181/#post-1373909

    Happy holiday.
     

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