Step cut Q's - diffs between crown height, depth, angle

[Lula]

I am so confused. I've been researching emerald cuts, in hopes of learning enough to buy one on the secondary market one day. I've read a lot of old threads, and Karl's p3 angle article. But one thing that I am confused about is the relationship between crown angle, crown height, and crown depth. For example, does a stone with a 36 degree crown angle *probably* have a >12% crown height? Or are the two numbers unrelated? Does the crown angle relate to the size of the table, with larger tables having lower crown angles? I'm asking because in the secondary market, so many diamonds have reports with only partial information. I'm getting pretty good at eyeballing stones for large, glassy tables, and shallow crowns (flat tops). On ebay, it's sometimes obvious if the sellers post profile views of the stones. I've seen some very odd depth-table percentage combos out there, too. Fancy cuts are a whole other ballgame.

Thanks

 

[stone-cold11]

crown height is crown depth.

For step cuts, as there are multiple step/angles in the cut, angles does not relates to crown depth and table.

 

[Lula]

crown height is crown depth.

For step cuts, as there are multiple step/angles in the cut, angles does not relates to crown depth and table.

Thanks, Stone. Here's an example of a stone I am considering:

This is what's listed on the vendor's website:

Depth % 75.69
Table % 65.38
Crown Depth 10.69%
Crown Angle 34.5

But when I look at the Helium report for the diamond, the crown height is listed as 15.85 (average for sides) and 16.14 (average for corners).

Is the 10.69 a typo? The Helium would be more accurate, right?

ETA: When a stone has a negative spread, does that mean it appears large for its carat weight?

 

[stone-cold11]

Probably but also depends on what measurement is used as a base for the %. Ask the vendor or post the reports here?

 

[Lula]

Thanks, Stone.
It's this GOG diamond. http://www.goodoldgold.com/diamond/6292/The cutting on this stone appears most bizarre. One side of the pavilion has three steps; the other side four steps. There appear to be extra facets on the crown -- or at least that's what the diagrams in the Helium report show -- making it look like there is an "incomplete" third set of crown facets. The table on the Helium diagram measures 72.3%. But on the GIA report, the table is given as 63%.

 

[John P]

Thanks, Stone.
It's this GOG diamond. http://www.goodoldgold.com/diamond/6292/The cutting on this stone appears most bizarre. One side of the pavilion has three steps; the other side four steps. There appear to be extra facets on the crown -- or at least that's what the diagrams in the Helium report show -- making it look like there is an "incomplete" third set of crown facets. The table on the Helium diagram measures 72.3%. But on the GIA report, the table is given as 63%.

Lula,

There are scan discrepancies. The biggest challenge in accuracy for any scanner is table width and primary crown-angle (as it relates to the table) since the diamond us lying upside down, with the table making contact with the scanner's platform...during the so-called "non-contact" scan. Any scanner, even the Helium, can find step cuts challenging. Add to that the small size of this diamond and you have a very hard candidate for scanning.

The GIA Report, GOG Helium and GOG Sarin all agree to a remarkably small tolerance on length, width and depth in mm. However it's clear that the Helium scan missed on the table/crown relationship (which also throws the depth% out the window). GOG may be able to re-run it, or they may already have tried and are limited by the diamond's size and configuration.

Both GOG's Helium and Sarin scans show at least one extra facet. None are mentioned by GIA so I suspect these are "ghost" facets (you saw this as "four steps" on the Helium) which would also indicate a difficult scan candidate. On the whole, GOG's local Sarin scan is closer to the GIA Report's basic data - but that table measurement is still 1.5% away from GIA's.

The GIA report is commonly considered the document of authority. The dossier report is logical for this diamond but gives very little information and no plot to verify the steps-against. I'd encourage you to speak to someone with the diamond in-hand for the questions you're raising... The nice thing here is that GOG is one of the few places on the planet that "speaks" our language of scans and angles fluently. Perhaps you can ring up Rhino to get the skinny on this little guy.

 

[Serg]

John,

re:The GIA Report, GOG Helium and GOG Sarin all agree to a remarkably small tolerance on length, width and depth in mm. However it's clear that the Helium scan missed on the table/crown relationship (which also throws the depth% out the window). GOG may be able to re-run it, or they may already have tried and are limited by the diamond's size and configuration.

This Helium 3D model is very bad. Without mmd file( with contours ) I can not understand reason so bad construction . Most probably Rhino used very old software ( report had been generated more than 2 years ago)

modern Helium software builds Emerald 3D models without significant problems. Emerald , Step cut are favorite Cut Types for Helium.

 

[Lula]

Thank you, John and Serg. Very informative posts. I will give Jonathan's staff a call in the morning about this diamond.

ETA: Good to know the cut on this stone is likely not as wonky as it appears! Ghost facets -- who knew?

 

[John P]

John,

re:The GIA Report, GOG Helium and GOG Sarin all agree to a remarkably small tolerance on length, width and depth in mm. However it's clear that the Helium scan missed on the table/crown relationship (which also throws the depth% out the window). GOG may be able to re-run it, or they may already have tried and are limited by the diamond's size and configuration.

This Helium 3D model is very bad. Without mmd file( with contours ) I can not understand reason so bad construction . Most probably Rhino used very old software ( report had been generated more than 2 years ago)

modern Helium software builds Emerald 3D models without significant problems. Emerald , Step cut are favorite Cut Types for Helium.

Thanks for the information Sergey. It's good to know the new software favors steps. What about size? Using the modern software how small would you say a stone like this could be (mm x mm x mm) with a solid 3D model build?

 

[John P]

Ghost facets -- who knew?

BOO!

(you're welcome)

 

[Serg]

John,
min size depends from FOV scanner.
Rhino used scanner with 8.6 mm FOV. 2mm diamonds are difficult for such FOV for several reasons( table of diamond is less than hole for pomp , some facets are less than 5 pixels,..). This emerald has 3.6mm-6mm diameter what is not problem at all for FOV 8.6mm.( 3.6mm is big enough, diagonal is just 7mm )
Now we have scanners with 4.4 mm FOV what is fine even for 2mm diamonds.

 

[Karl_K]

Sorry I'm late to this thread but it looks like your getting great help from John and Serg.
I agree bad scan.

It is very deep for that table size so it faces up a bit small.
The large leakage stripe in the IS and ASET on one side bugs me.

If your interested in it.
Ask Jon if he has a video with this diamond in it, in particular you want to see how it handles tilt.

 

[John P]

John,
min size depends from FOV scanner.
Rhino used scanner with 8.6 mm FOV. 2mm diamonds are difficult for such FOV for several reasons( table of diamond is less than hole for pomp , some facets are less than 5 pixels,..). This emerald has 3.6mm-6mm diameter what is not problem at all for FOV 8.6mm.( 3.6mm is big enough, diagonal is just 7mm )
Now we have scanners with 4.4 mm FOV what is fine even for 2mm diamonds.

It all sounds logical Serg. Can I ask, what is the maximum size (mm) that can be reasonably scanned - with the 8.6, and also the 4.6 FOV versions?

 

[John P]

Sorry I'm late to this thread but it looks like your getting great help from John and Serg.
I agree bad scan.

It is very deep for that table size so it faces up a bit small.
The large leakage stripe in the IS and ASET on one side bugs me.

If your interested in it.
Ask Jon if he has a video with this diamond in it, in particular you want to see how it handles tilt.

Especially true with step cuts, where the windowing we see may or may not be as dire as it can appear in a static ASET photo, especially with backlighting. I sent an email to Rhino about this thread. Hopefully he can take a minute to chime in. I still encourage you to reach out to him Lula, as much more can be covered in a phone call.

 

[Lula]

Thanks, Sergey and John for the addition information about the scans. Several of the vendors provide these scans on their websites and I think some discussion on how to read the scans, scan error, etc., is useful for consumers. I've only looked scans for rounds before, so the scan for this little emerald-cut stone had me completely baffled. So the information about scan error was enlightening.

I put the stone on hold and e-mailed Good Old Gold about it. I'll let you know what I hear back from them.

Karl, thank you for your input. Yes, I was wondering about that dark and prominent bar that shows up in both the ASET and the IS. The stone appears tilted in all the photos, so I was having a hard time getting a sense of the facet pattern. I would have never thought to ask about how the stone handles tilt, so thanks for that comment. I've long wanted a step-cut stone, but the learning curve is quite steep. At first, they all looked pretty much the same to me, especially under store lighting.

However, after looking at the emerald-asscher thread over in SMTB, and doing more reading about facet patterns, now when I look at emerald and asscher cuts in local pawn/second-hand shops, I'm getting better at knowing what to look for on visual inspection. Some emerald cuts show a distinct dark "cross" face-up. Some show no obstruction, but show no depth or hall of mirrors effect either. They have a flat, open look to them -- I'm curious if the flat look is due to light leaking out the sides and bottom of the stone? Ebay is a minefield of bad photography and over-priced stones. Cringe.

I tend to like the elongated emerald-cuts (the look more Art Deco to me) or the more squat 1.2 *ish* ratios. As for depth, I would have ruled out this GOG stone, except I found this old thread https://www.pricescope.com/community/threads/why-are-many-emerald-cuts-too-deep.76190/ which made me curious about the effect of depth (other than on spread) combined with a smaller (63%) table on the optics of the stone.

 

[Serg]

John,
min size depends from FOV scanner.
Rhino used scanner with 8.6 mm FOV. 2mm diamonds are difficult for such FOV for several reasons( table of diamond is less than hole for pomp , some facets are less than 5 pixels,..). This emerald has 3.6mm-6mm diameter what is not problem at all for FOV 8.6mm.( 3.6mm is big enough, diagonal is just 7mm )
Now we have scanners with 4.4 mm FOV what is fine even for 2mm diamonds.

It all sounds logical Serg. Can I ask, what is the maximum size (mm) that can be reasonably scanned - with the 8.6, and also the 4.6 FOV versions?

it depends from Cut( maximum geometrical diameter) and time what you agree to spend for centering diamond on holder. Theoretically you can scan 8.5 mm round, practically you prefer do not scan by Helium 8.6mm any diamond bigger than 8mm ( diamond with biggest geometrical diameter 8mm). Of course you can not scan Emerald with Length ( max diameter in Gemological reports) =8mm and Width=4mm because diagonal ( max geometrical diameter) is 8.94mm what is higher than 8.6mm.

 

[Lula]

Hi -- Just updating this thread. I've spoken to a rep at GOG. They are going to take a look at the stone and answer my questions about crown height (GIA facet ware # is different from # on GOG's webpage). I also asked them to look at how the stone handles tilt per Karl's suggestion. I may also ask them to call in a few other emerald cuts for comparison.

Thanks again, everyone, for an interesting discussion on Helium analysis.