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 Post subject: Need help, to recut a dark spinel. How to brighten?
PostPosted: Fri Feb 16, 2007 12:35 am 
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Got a 22 carat dark red with pink overtone spinel from srilanka. The overall tone is similar to some very nice stones I have, but a shoddy native cut and too much depth are hiding its beauty and sparkle. I am going to recut it as close to critical angle as possible, I was wondering if anyone could recommend any cuts. The stone is currently a squarish cushion cut with a high crown. At the moment I'm thinking something like a fancy princess style cut, but would love to hear any advice anyone has, Basically I want to get the darkness down and the colour/sparkle up.

Thanks in advance - Mick

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PostPosted: Sat Feb 17, 2007 7:39 am 
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Picture(s) ? ... and the basic dimensions...

Only if it isn't trouble. I am not a cutter. You might find the follow up fun though 8)

For what that matters, your description of the stone so far brings to mind a very classic shape... which I believe to be quite popular too.


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 Post subject: design
PostPosted: Sat Feb 17, 2007 8:19 am 
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Mick,
Doug turret modified his dark star design for me from an srb to a cushion. This is his design in lapidary journal to lighten up dark stones.. I did an aussie saphire in this design, but i need more cutting tests to decide how good it is.. I'll check with him and see if i can send it to you..


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PostPosted: Sat Feb 17, 2007 3:42 pm 
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Thanks very much for that mate, its much appreciated.

I will take a photo of the stone on monday as I'm flying out today and the camera is packed.

All the best - Mick

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PostPosted: Tue Feb 20, 2007 12:37 pm 
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May I ask... in general... are critical angles written in stone regardless of the darkness of the material? (esp. any chance a tad less would fit higher absorption... when a reflector pavilion could light things up w/o windowing and a button type of crown might make some of the tilt window look good). Basically, just curious... how well a model in the hands of a by-stander might work to predict reality.

For clarification: I am not giving advice (if anyone might read this as such :oops: ), just hoping for a lighthearted (SIC!) discussion.


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 Post subject: Re: design
PostPosted: Tue Feb 20, 2007 8:23 pm 
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I'm curious, did you post pics of that stone somewhere? I don't think I've seen that design; I don't subscribe to LJ and it's hard to find on the newstand sometimes. If you have something you could share, I'd love to have a look.

jleb wrote:
Mick,
Doug turret modified his dark star design for me from an srb to a cushion. This is his design in lapidary journal to lighten up dark stones.. I did an aussie saphire in this design, but i need more cutting tests to decide how good it is.. I'll check with him and see if i can send it to you..

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PostPosted: Tue Feb 20, 2007 8:43 pm 
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Hi Valeria,
Even if critical angle for a species of stone is a given, there is some research out that indicates that there can be a broad range of good cutting angles for that material. There is a site that has some good information published: http://www.gemology.ru/cut/english/faceting/

I sometimes use the charts on that site, along with Gemcad software, to predict what might happen with a design before cutting it. Together they help determine things like what sort of crown or pavilion works best, but the one thing they don't account for is saturation of the stone. Sometimes a design with a high light return can lighten a dark stone, sometimes not in my experience. The tools help manage the tradeoffs such as tilt performance and such, but I never know for sure what will happen until the stone is cut. It's a good question you ask, and I'd like to hear what others think about it.


valeria102 wrote:
May I ask... in general... are critical angles written in stone regardless of the darkness of the material? (esp. any chance a tad less would fit higher absorption... when a reflector pavilion could light things up w/o windowing and a button type of crown might make some of the tilt window look good). Basically, just curious... how well a model in the hands of a by-stander might work to predict reality.

For clarification: I am not giving advice (if anyone might read this as such :oops: ), just hoping for a lighthearted (SIC!) discussion.

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PostPosted: Wed Feb 21, 2007 4:29 am 
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HME wrote:

... There is a site that has some good information published: http://www.gemology.ru/cut/english/faceting/

...the one thing they don't account for is saturation of the stone. Sometimes a design with a high light return can lighten a dark stone, sometimes not in my experience.



They do account for saturation - their DiamCalc software lets you adjust RI and saturation before tweaking angles. Light color, intensity, direction, focus can change to simulate a 'setting' (pavilion in the dark) etc. Btw: ignore the 'diam' - it allows any RI to be modeled, only thing that works only for diamonds is a cut score. I have a HASP version of it to play with - it can make a dark spinel material look like this: the 'cubes' below had absorbtion added manually to the colorless default with spinel RI:

Image

The slight color difference is given by a change in light temperature while keeping absorbtion the same. Making the volume thicker makes it look darker, as expeted.

If anyone has a spectrum for dark red spinel, this could get quite realistic, methinks.

The virtual spinel material can be 'cut' and the results optimized under different lighting, using ray tracing or virtual reflector devices.


Apologies in advance if this wasn't cool... I wouldn't ever pretend that proficiency with a bit of software could possibly compensate for real experience. The little toy made me respect the intricacies of cutting, if anything. Hope it is the same for anyone who might do the same.


Last edited by valeria102 on Wed Feb 21, 2007 1:45 pm, edited 1 time in total.

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PostPosted: Wed Feb 21, 2007 1:04 pm 
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Not being familiar with DiamCalc, I didn't realize it allows for saturation levels. I'll have to look into that software. Thanks!

valeria102 wrote:
HME wrote:



They do account for saturation - their DiamCalc software lets you adjust RI and saturation before tweaking angles. Light color, intensity, direction,


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PostPosted: Thu Feb 22, 2007 12:03 am 
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You need to do a shallow cut. Overally dark stones will always look dark, so what you want to do is minimize the distance the light travels.
Larger facets tend to help, as do checker board crowns.

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PostPosted: Thu Feb 22, 2007 6:22 am 
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Concave faceting wasn't supposed t work in the dark, or was it (Gene?) ?

Here's why I am asking: LINK (~4.5cts, dark tsavorite, ~11 x 5mm, concave facets on pavilion). It looks like those might have been used to close a window ... but, what do I know.

A big, dark red brother of that might look great in the sort of modernistic thing Stuart Mooremight carry. That large tsavorite has been sitting around for a while all by itself, if memory serves.


Last edited by valeria102 on Thu Feb 22, 2007 6:34 am, edited 1 time in total.

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PostPosted: Thu Feb 22, 2007 6:34 am 
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I remember someone saying that concave actually darkens the stones.


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 Post subject: concave
PostPosted: Thu Feb 22, 2007 9:35 am 
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Concave usually does darken the stone. I think with the tsavorite posted above, the stone could have been brighter had it not been concave cut. The stone doesn't look shallow, so I don't think the intention was to remove a window. From my experiance cutting concave, you still need to pay attention to the critical angles. A concave cut stone will still window.
I havn't concave cut anything in a few weeks, maybe I should do one next. To me the concave stones loose some of their personality, and it does add about another hour or more on to the cutting time.

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 Post subject: Re: concave
PostPosted: Thu Feb 22, 2007 9:51 am 
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Precision Gem wrote:

The stone doesn't look shallow, ....



Yeah... that's the point: the thing is 50% deep (oups. that link doesn't give any measurements, there are a few more details in its full decsription HERE)


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PostPosted: Thu Feb 22, 2007 2:09 pm 
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Well, that tsavorite just looks overcolored and too dark. Maybe if it were recut with a shallow pavilion right around the critical angle, and less facets it may perk up a bit. My experiance with dark rough is, that it never cuts well. If you have to shine a flash light into to see decent color, then it's too dark.
I suspect at the price of that garnet, it will sit around for a long time.

Someday, when I have nothing to do, I may cut two identical stones from identical material (lab), one with traditional faceting, and one with concave. I'll keep the angles the same, and see how the face up.

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