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One would think that it would be easy to find data on emerald cut
emeralds, but Web searches yields almost entirely emerald cut
*diamonds*. I have to specifically exclude the word "diamond" from the
search in order for the Web to acknowledge that any other gems exist on
this planet other than diamonds.
But even so, it seems impossible to find information on cut angles for
emerald. Lots of descriptions of the emerald cut, but no actual
numbers. I did find one informal recommendation for an emerald cut--for
sapphire! But sapphire's IOR is considerably higher than emerald's.
So, I tried finding some viable angles myself. I'm not having any luck;
all but the shallowest crowns leak light like a sieve:
emerald_specs-flat.png
Demonstrates the range of pavilion angles that contain the light
with a flat crown.
emerald_specs-near_flat.png
Any crowns steeper than this will leak light.
emerald_specs-shallow.png
A damn shallow crown.
emerald_specs-steep.png
And it doesn't get any better at steeper angles.
I used an IOR of 1.584, which is what I estimate is emerald's IOR at a
green wavelength, based on 1.580 at D and a dispersion of 1.089.
I went searching deeper, and found a cut designed by Jim Perkins, and
published in Lapidary Journal Jewelry Artist, October 2010. Although
his cut is not a traditional step cut, I tested his angles in my rig:
emerald_specs-perkins-long.png
The angles Perkins used on the long side of his cut.
It still leaks, even though he calibrated his cut for an IOR of 1.54,
which I imagine less forgiving than emerald's. Is it possible to get a
leakless emerald cut with a decent crown? Or do they count on leaked
light being reflected back from the setting? Will light coming into the
crown at an angle leak less, or at least compensate? (I suppose I can
find out if I budget more render time.) Can someone channel Steve Paget?
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Attachments:
Download 'emerald_specs-flat.png' (74 KB)
Download 'emerald_specs-near_flat.png' (73 KB)
Download 'emerald_specs-shallow.png' (70 KB)
Download 'emerald_specs-steep.png' (79 KB)
Download 'emerald_specs-perkins-long.png' (79 KB)
Preview of image 'emerald_specs-flat.png'
Preview of image 'emerald_specs-near_flat.png'
Preview of image 'emerald_specs-shallow.png'
Preview of image 'emerald_specs-steep.png'
Preview of image 'emerald_specs-perkins-long.png'
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On 2017-08-12 11:18 PM (-4), Cousin Ricky wrote:
> emerald_specs-flat.png
> Demonstrates the range of pavilion angles that contain the light
> with a flat crown.
> emerald_specs-near_flat.png
> Any crowns steeper than this will leak light.
> emerald_specs-shallow.png
> A damn shallow crown.
> emerald_specs-steep.png
> And it doesn't get any better at steeper angles.
These are rendered with the light source slightly offset, because in
real life, lights directly behind us don't shine through our skulls.
The stones are tilted 45 degrees from the vertical, with the camera
positioned perpendicular to the table.
test_gemcuts-emerald-face.jpg
Has the same cross section as emerald_specs-steep.png
test_gemcuts-emerald-near_flat.jpg
Has the same cross section as emerald_specs-near_flat.png. No light
leakage, but it looks pretty pathetic.
Post a reply to this message
Attachments:
Download 'test_gemcuts-emerald-face.jpg' (13 KB)
Download 'test_gemcuts-emerald-near_flat.jpg' (12 KB)
Preview of image 'test_gemcuts-emerald-face.jpg'
Preview of image 'test_gemcuts-emerald-near_flat.jpg'
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On 8/13/2017 4:18 AM, Cousin Ricky wrote:
> One would think that it would be easy to find data on emerald cut
[Snip]
>
> It still leaks, even though he calibrated his cut for an IOR of 1.54,
> which I imagine less forgiving than emerald's. Is it possible to get a
> leakless emerald cut with a decent crown? Or do they count on leaked
> light being reflected back from the setting? Will light coming into the
> crown at an angle leak less, or at least compensate? (I suppose I can
> find out if I budget more render time.) Can someone channel Steve Paget?
I remember the time when people would shout at you for being on topic in
off topic. ;)
Looking at the site below. It looks to me that a 2D test will not
represent a 3D object very well as for the pavilion, the Break Facets
are rotated and not in the same plane as the adjacent Pavilion Mains.
Actually I get the feeling that you might need to do two or three
images. One for the Crown Mains with the Pavilion Mains and one with the
Crown Break Facets with the Pavilion Break Facets.
I assume that three angles are used in an emerald cut as opposed to two
for a brilliant cut?
I've got a gut feeling that you need to do them separately.
https://www.gemsociety.org/article/lapidary-fundamentals-gemstone-faceting/
--
Regards
Stephen
Post a reply to this message
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On 2017-08-13 11:39 AM (-4), Stephen wrote:
> Looking at the site below. It looks to me that a 2D test will not
> represent a 3D object very well as for the pavilion, the Break Facets
> are rotated and not in the same plane as the adjacent Pavilion Mains.
> Actually I get the feeling that you might need to do two or three
> images. One for the Crown Mains with the Pavilion Mains and one with the
> Crown Break Facets with the Pavilion Break Facets.
> I assume that three angles are used in an emerald cut as opposed to two
> for a brilliant cut?
>
> I've got a gut feeling that you need to do them separately.
The emerald cut is quite different from the round brilliant. There are
three crown facet levels, three pavilion facet levels, and no break
facets. A 2-D representation is quite adequate for the middle part of
this cut.
> https://www.gemsociety.org/article/lapidary-fundamentals-gemstone-faceting/
The Web is replete with many different gem cuts and materials, but seems
to have the particulars for only one cut and only one material. The
particulars on this page are for the round brilliant cut, and the stated
angles indicate that the material they have in mind is diamond: exactly
the information that is ubiquitous all over the Web, and therefore
exactly *not* what I'm looking for. (Whoever said diamonds are rare
either hasn't done a Google search or works for De Beers' marketing
department.)
Where it says that the procedures are the same for any material, they
are referring to the technique, not the actual numbers that you plug
into the machine. And the technique is what I'm least interested in
(for now), because I'm trying to simulate a gemstone, not cut one.
(Hmmm, I see you point. This probably isn't off-topic after all.)
Sorry.
Post a reply to this message
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On 8/13/2017 7:00 PM, Cousin Ricky wrote:
> On 2017-08-13 11:39 AM (-4), Stephen wrote:
>> Looking at the site below. It looks to me that a 2D test will not
>> represent a 3D object very well as for the pavilion, the Break Facets
>> are rotated and not in the same plane as the adjacent Pavilion Mains.
>> Actually I get the feeling that you might need to do two or three
>> images. One for the Crown Mains with the Pavilion Mains and one with the
>> Crown Break Facets with the Pavilion Break Facets.
>> I assume that three angles are used in an emerald cut as opposed to two
>> for a brilliant cut?
>>
>> I've got a gut feeling that you need to do them separately.
>
> The emerald cut is quite different from the round brilliant. There are
> three crown facet levels, three pavilion facet levels, and no break
> facets. A 2-D representation is quite adequate for the middle part of
> this cut.
>
I'll take your word for it.
>> https://www.gemsociety.org/article/lapidary-fundamentals-gemstone-faceting/
>>
>
> The Web is replete with many different gem cuts and materials, but seems
> to have the particulars for only one cut and only one material. The
> particulars on this page are for the round brilliant cut, and the stated
> angles indicate that the material they have in mind is diamond: exactly
> the information that is ubiquitous all over the Web, and therefore
> exactly *not* what I'm looking for. (Whoever said diamonds are rare
> either hasn't done a Google search or works for De Beers' marketing
> department.)
>
They're so common they even use them in drill bits.
> Where it says that the procedures are the same for any material, they
> are referring to the technique, not the actual numbers that you plug
> into the machine. And the technique is what I'm least interested in
> (for now), because I'm trying to simulate a gemstone, not cut one.
> (Hmmm, I see you point. This probably isn't off-topic after all.)
>
> Sorry.
>
I'm sorry I couldn't help.
--
Regards
Stephen
Post a reply to this message
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