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OEC Ideal Cut?

Karl_K

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Karl, let me see if I can correctly interpret what you wrote. :)

Given that Bruce was mostly interested in colored stones, and he made no mention of LGFs in the article, the odds are he did not take them into account when he made the diamond chart. For OECs shorter LGFs do not protrude under the table, and everything under the table is at the nominal PA. So for OECs the chart should be accurate. Am I right?
Mains are mains be it oec or mrb.
 

prs

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@prs I have *thoroughly* enjoyed your very thoughtful and very persistent :lol: analysis of old cut combinations. I have a transitional cut 60/60 stone in my engagement ring, and -- since you seem to be trending toward that sort of combination for DW, though I think the LGFs on mine are a little longer than Dreamer's/Arkie's -- I wondered if seeing a close-up photo of it with its numbers (EGL cert, fwiw) would be helpful for you. It certainly would be interesting for me to hear how you break down the numerical basis for what you see. Happy to post them if that isn't an unwelcome detour, otherwise, I'll just keep following along with interest! :D
The whole reason for this thread is our lack of useful information. If GIA certs gave us the cut data, we could all easily figure out what proportions produce the best looking stones. So the answer to your question is: YES PLEASE!!! :mrgreen2: Another good data point would be very, very welcome.
 

Karl_K

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While its possible to have a combo where the lowers show obstruction if there is no twist present you wont find an example.
When they are twisted in 3d space all bets are off.
 

evergreen

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@prs -- My diamond was even shallower than I remembered. :lol: These "rule-breakers" make your quest particularly interesting!

Let me know if these photos are adequate for measurement purposes. I tried several different techniques, but am still not entirely happy...

IMG_20200712_192551.jpg
DSC01283_1.JPG DSC01289_1.JPG IMG_20200712_193145.jpg
(The last one was taken through a loupe with my white phone, the others with a black camera; the "obstruction" from the black camera is important for highlighting arrows, I guess!)
 

prs

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Here are a couple of very interesting excerpts from Bruce Harding's faceting article:

BH Pg 1.png BH Pg 3.png

And here is his faceting chart for diamond with the three possible areas of good proportions marked A, B, and C.

BH Chart Diamond 2A.jpg

Of course the proportions of MRBs and almost all OECs fall into area A. @evergreen I'm still working on this, but I think the proportions of your stone are very similar to dreamers's OEC. I also think your "rule breakers" might actually fall into area B, so maybe not "rule breakers" after all. The cutter was operating outside the box.
 

Karl_K

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BH Chart Diamond 2A.jpg

Of course the proportions of MRBs and almost all OECs fall into area A. @evergreen I'm still working on this, but I think the proportions of your stone are very similar to dreamers's OEC. I also think your "rule breakers" might actually fall into area B, so maybe not "rule breakers" after all. The cutter was operating outside the box.
A,B and C are not for diamond.
Those also only consider the mains and only obstruction.
When everything is considered the dot is at the mrb range.
 

Karl_K

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@prs -- My diamond was even shallower than I remembered. :lol: These "rule-breakers" make your quest particularly interesting!

Let me know if these photos are adequate for measurement purposes. I tried several different techniques, but am still not entirely happy...

IMG_20200712_192551.jpg
DSC01283_1.JPG DSC01289_1.JPG IMG_20200712_193145.jpg
(The last one was taken through a loupe with my white phone, the others with a black camera; the "obstruction" from the black camera is important for highlighting arrows, I guess!)
fairly close but with more symmetry.

close.jpg
 

evergreen

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fairly close but with more symmetry.

close.jpg
Whoa, very cool! To be honest, it looks really unimpressive in that image -- how interesting, and now I am questioning my ability to interpret those white-and-grey images! What would you conclude about the diamond based on that image? Is that a...fish-eye? :???:I know shallower diamonds can be susceptible to that.
 

Karl_K

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Whoa, very cool! To be honest, it looks really unimpressive in that image -- how interesting, and now I am questioning my ability to interpret those white-and-grey images! What would you conclude about the diamond based on that image? Is that a...fish-eye? :???:I know shallower diamonds can be susceptible to that.
Its the lighting enviirement and normal.
I forgot to save the file I will redo and run a fish-eye image for it.
I suspect there may be some but not enough to hurt it.
It is one of those wierd combos that work.
 

prs

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A,B and C are not for diamond.
Those also only consider the mains and only obstruction.
When everything is considered the dot is at the mrb range.
Of course the dot is in the MRB range, it's in area A and at the Tolkowsky ideal proportion, as stated in the article. You might want to read the article, it's very interesting!

Here's the AGS cut guide with a more complete Harding chart that includes a portion of area B

AGS Cut Guide 10mm MRB 50% Table Rev C.jpg
 

prs

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fairly close but with more symmetry.

close.jpg
There's something strange about your numbers here. According to my calculations a pavilion depth of 40% together with a medium 3% culet should give a PA of 39.6°. Your calculation is giving 38.94°. GIA Facetware also gives 39.6°. I wonder if your calculation doesn't take the culet into account?
 
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Karl_K

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it's in area A

AGS Cut Guide 10mm MRB 50% Table Rev C.jpg
Notice how per ags rules how many unsuitable combinations are in A.
They may be fine from obstruction off the mains but other factors are not optimal.
 

Karl_K

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There's something strange about your numbers here. According to my calculations a pavilion depth of 40% together with a medium 3% culet should give a PA of 39.6°. Your calculation is giving 38.94°. GIA Facetware also gives 39.6°. I wonder if your calculation thinks the culet is 6% of diameter rather than 3%?
Here are the % used by the software.
Given the lose physical symmetry / tightness of the angles in relation to each other of the original and the rounding of the numbers its never going to be exactly correct.
percentage.jpg
 
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Karl_K

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As I thought by looking at the virtual images and your pictures there is a small amount of fish-eye present.
The fish-eye are areas where light is present.
In the virtual diamond it looks like this.
fisheye.jpg
My opinion is the same beautiful diamond with odd proportions that work.
 

evergreen

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Thanks, @Karl_K -- way more than I knew about my diamond's cut before @prs undertook this adventure! Finding sweet spots in a region of shallow cuts is a nice aim, if you can get rough that's appropriate for them -- we often talk about how the most visible characteristic is light return, but then after that, it's *size*, so I've always been pleased at this diamond's face-up size for its weight. My MIL got a ~2ct upgrade (without consulting me :cry2: so, no sense in discussing cut quality!) and my 1.30ct, admittedly bezeled, looks the same size as her 2ct.
 
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prs

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The culet in @evergreen's diamond actually measures out at 7%, large on the GIA culet scale. This results in a PA of 40.6°.
 

Karl_K

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The culet in @evergreen's diamond actually measures out at 7%, large on the GIA culet scale. This results in a PA of 40.6°.
Lets check that and learn together.
doesn't match close enough.
the pavilion is shallower than 40.6
. 406p.jpg
edit: to make it sound more like I intended it..
 

prs

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Lets check that and learn together.

Sure, it's easy to check. Print out this photo of the diamond and use a ruler to measure the width of the culet vs the width of the diamond. On my printer it comes out at 9mm vs 135mm or 6.67%. :)

Screenshot (586).png
 

Karl_K

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Sure, it's easy to check. Print out this photo of the diamond and use a ruler to measure the width of the culet vs the width of the diamond. On my printer it comes out at 9mm vs 135mm or 6.67%. :)

Screenshot (586).png
I believe ya on that, but 40.6 does not work therefor there is an issue someplace.
 

Karl_K

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Sure, it's easy to check. Print out this photo of the diamond and use a ruler to measure the width of the culet vs the width of the diamond. On my printer it comes out at 9mm vs 135mm or 6.67%. :)

Screenshot (586).png
near as I can line it up 6.2ish in DC.
To much variation to nail the pavilion angle that way,
 

Karl_K

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6.2% comes out as 40.46 degrees which is also not shallow enough. Are you starting to doubt the report numbers?
Some rounding going on?
4046.jpg
 

evergreen

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Sure, it's easy to check. Print out this photo of the diamond and use a ruler to measure the width of the culet vs the width of the diamond. On my printer it comes out at 9mm vs 135mm or 6.67%. :)
I've thrown an additional wrench in by bezeling the stone -- you may need to measure vs the table and back-calculate since some undetermined amount of girdle is covered by the bezel. Or maybe that isn't it and you have already adjusted -- you are a very precise person, @prs, and it would not surprise me. :D
 
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prs

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6.2% comes out as 40.46 degrees which is also not shallow enough. Are you starting to doubt the report numbers?
Some rounding going on?
Screenshot (588).png

According to GIA a "Very Thin to Thin" girdle should measure no more than 1.6%. so add 40.0+13.9+1.6 and we get 55.5%. However the actual depth is 56.7%, assuming EGL know how to use a pair of calipers! :mrgreen2:

So yes, we can start to doubt the numbers given we have a missing 1.2%. :eek-2:
 

Karl_K

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Screenshot (588).png

According to GIA a "Very Thin to Thin" girdle should measure no more than 1.6%. so add 40.0+13.9+1.6 and we get 55.5%. However the actual depth is 56.7%, assuming EGL know how to use a pair of calipers! :mrgreen2:

So yes, we can start to doubt the numbers given we have a missing 1.2%. :eek-2:
Oh no its Warped! (not really)
Actually they are measured from different points is why they dont add up.
Do a google search for warped diamonds pricescope
 

prs

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@prs -- My diamond was even shallower than I remembered. :lol: These "rule-breakers" make your quest particularly interesting!

Let me know if these photos are adequate for measurement purposes. I tried several different techniques, but am still not entirely happy...

IMG_20200712_192551.jpg
DSC01283_1.JPG DSC01289_1.JPG IMG_20200712_193145.jpg
(The last one was taken through a loupe with my white phone, the others with a black camera; the "obstruction" from the black camera is important for highlighting arrows, I guess!)
The facet pattern on your diamond is terrific. The stars align with the crown, and I estimate the LGFs to be about 65%. Of course with a 57% table that makes them more visible than if the table was smaller, but that's just fine with me. :love: I'm sorry but I don't feel confident enough in the numbers on your report to estimate the CA and PA angles.

I've never actually seen obstruction in person, so most likely I don't know what I'm talking about, but I do wonder in your photos if the dark areas under the table might be caused by obstruction?
 

evergreen

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I'm sorry but I don't feel confident enough in the numbers on your report to estimate the CA and PA angles.
Is EGL enough of a 3rd rate lab to have gotten them wrong?? Totally could be! I need to reset the stone and I was going to send it to GIA while unset to get a "real" report -- it could be a while, a year or more even, but I'll update when I do! It's extra interesting that empirically calculating the values based on the photo isn't yielding expected results.

And re: obstruction -- yes, the "arrows" are the result of obstruction, if that's what you mean by dark areas. Some obstruction is necessary for visible patterning! It's a strategy when taking photos to selectively give your diamond something in the "obstruction" range that you'd like reflected directly to the viewer. To get the rainbow chunky sparkles photos, something like filtered sunlight through a tree works well; to get arrows,a black camera is ideal (and a white camera, like the back of my cell phone in the loupe photo, will not yield such clear dark/bright areas). Is that what you were commenting on, though, or have I just meandered off the subject? :)
 

prs

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I've thrown an additional wrench in by bezeling the stone -- you may need to measure vs the table and back-calculate since some undetermined amount of girdle is covered by the bezel. Or maybe that isn't it and you have already adjusted -- you are a very precise person, @prs, and it would not surprise me. :D
To be honest I just made a guess at the width of your diamond. More of a plodder than precise, but most times I do somehow manage to get to the right answer. :)

Is EGL enough of a 3rd rate lab to have gotten them wrong?? Totally could be! I need to reset the stone and I was going to send it to GIA while unset to get a "real" report -- it could be a while, a year or more even, but I'll update when I do! It's extra interesting that empirically calculating the values based on the photo isn't yielding expected results.
Unfortunately GIA do not provide cut detail for OECs or Circular Brilliants. It's possible they might treat your diamond as an MRB, and only then would we learn the true proportions.

The estate jewelry trade has sold the public on the fact it's the high crowns that make OECs so beautiful, and that's why their depths are so much greater, and their spreads so much lower than a same carat weight MRB. What they conveniently forgot to mention is that many OECs also have steep pavilions. These deep pavilions add nothing to beauty, in fact they hurt light return. but for the seller they hide a heck of a lot of carat weight. IMHO this is the reason the trade keeps the cut details hidden from the public.
 
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prs

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And re: obstruction -- yes, the "arrows" are the result of obstruction, if that's what you mean by dark areas. Some obstruction is necessary for visible patterning! It's a strategy when taking photos to selectively give your diamond something in the "obstruction" range that you'd like reflected directly to the viewer. To get the rainbow chunky sparkles photos, something like filtered sunlight through a tree works well; to get arrows,a black camera is ideal (and a white camera, like the back of my cell phone in the loupe photo, will not yield such clear dark/bright areas). Is that what you were commenting on, though, or have I just meandered off the subject? :)
Not meandering at all, that is exactly what I was commenting on. Thanks for the very helpful explanation. :)

I obviously need to know much more about obstruction, does this diamond learning thing never end???? :lol-2:
 

evergreen

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Ah, curses, I hadn't thought about the "circular brilliant" designation and its boring lack of information. Eyeballing it, it does look like the star length is <50%, and the culet will be medium or larger; with your LGF estimate of 65% it could go either way depending on the direction of uncertainty! Must mean I need to make friends with someone that has a Sarin or Helium scanner. ;-)

BTW, some obstruction is good/necessary, but too much is "bad". This is too much :shock: : https://www.jamesallen.com/loose-di...m-color-vs2-clarity-excellent-cut-sku-6385670 (but, y'know, since it's a 13mm diamond I wouldn't kick it out of bed for eating crackers.). You can/should imagine that, when looking directly at that diamond, it's going to reflect a lot of your face and not the ambient light.

I thought a lot about obstruction when looking for an emerald cut, since there are only like 15 virtual facets to think about. :lol: Contrast these four:
1594932110790.png
Just going VERY quickly through JA's inventory, here are 4 different-looking cuts. The first might have too much obstruction to be awesome, but then again, I just told you that obstruction is important for patterned flashing so it might be OK (I ultimately selected an emerald cut diamond from JA which looked a little over-obstructed in this view and I have no regrets!). B has a big ol' window with a lousy cut. C looks to me to have a good combination of obstruction and light return. And D lacks contrast -- to my eye, it needs *more* obstruction to give the patterned scintillation I expect. HTH. :)

link that might work to try to play with all 4 in their rotating view.
 
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