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 Post subject: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 12:52 am 
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I have a particularly tasty piece of flawless purple sapphire rough, rectangular in plan (table) view, and square in cross section. The best colour, a rich, deep purple, is seen through a rectangular face of the crystal. Perpendicular to this, along the other axes, the colour is a less saturated purple. A cut stone would inevitably be deep enough to appear almost black, with deep purple flashes. My thinking is to tilt the material by say 30 degrees so that the light path is more through the lighter purple crystal directions. Might this cause colour banding? Any advice or suggestions gratefully received. It's not exactly a cheap piece of material btw, so I can't afford to experiment.


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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 3:01 am 
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salik119 wrote:
It's not exactly a cheap piece of material btw, so I can't afford to experiment.


Sure you can, just not on that piece of rough. Get a lower cost sapphire with a dark blue axis on E-bay or one of the rough sellers in this country and try your ideas on that first.

The same thing happens with many saturated sapphires or tourmalines in that they will have a 'C' axis that is very dark. If the dark axis is going to be really dark regardless of the length that light traverses through it, (think "closed C" tourmaline), then you're better off avoiding that axis and cutting an emerald cut, smith bar, etc. If the dark axis can tolerate some light transmission, then I'd put the table directly on the lightest axis and "experiment" with the cut by using GemCad to "shoot" light rays through the stone in various places and see how much light is actually going through the dark axis from different entry points and angles.

If it looks like what I'm thinking, then I'd cut an oval or rectangular cushion with the table on a light axis so that light going across the short side of the oval/cushion is going through the dark axis. This way you minimize the light path through the dark axis and maximize it through the light axes. This should make the long ends of the oval/cushion lighter and short sides of the oval/ cushion darker, (but not too dark).

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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 5:30 am 
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Thanks Michael. Yes, I'd thought of that too. I hadn't considered orienting the material along a light axis though. It might be a bit too light. The best yield would probably come from about 45 degrees to the C axis, which might work. The rough is ideally suited to a rectangle or cushion rectangle, and it would be great to optimize the colour. I can certainly experiment with another piece of sapphire and different table angles wrt the C axis.


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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 11:51 am 
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Do you know how to do the "small mirror test"? Basically, if you put a piece of rough on a small mirror and put it in diffuse lighting, the color that the stone appears will be closest to the gem's final color. Light enters the stone, bounces off the mirror, passes back through the stone, and to your eye - this mimics light entering the stone, bouncing off the pavilion, and returning to your eye.

You can use this test to your advantage - rotate the stone around over the mirror and see if you can find an orientation that makes it look the color you want.


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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 1:03 pm 
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Hi Arya. Yes, I'm quite familiar with the mirror technique, particularly for rough selection and non-dichroic material. The difficulty with this type of sapphire is in predicting colour behaviour for totally internally reflected rays. My question could perhaps be re-phrased as follows:
Suppose you have a piece of sapphire which has strongly saturated colour along the C axis, and is clear along the other two orthogonal axes. If the table is oriented at an angle of, for example, 45 degrees to the C axis, what effect will this choice have on the colour saturation of the finished stone? Is there a "better" angle? GemRay sadly doesn't help here, hence the need to resort to experimentation.


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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 2:23 pm 
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Any chance you'd share a picture? It's not often I get to see nice purple sapphire (at least in facet grade) and it would be a sight for sore eyes. It might also help with people's suggestions, but I mostly just want to see it :)

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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 3:16 pm 
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salik119 wrote:
My question could perhaps be re-phrased as follows:
Suppose you have a piece of sapphire which has strongly saturated colour along the C axis, and is clear along the other two orthogonal axes. If the table is oriented at an angle of, for example, 45 degrees to the C axis, what effect will this choice have on the colour saturation of the finished stone? Is there a "better" angle? GemRay sadly doesn't help here, hence the need to resort to experimentation.

Ahhh, your rephrased question makes more sense now.

From my admittedly limited experience in this respect, I've found that the final color doesn't necessarily yield the sum of component vectors along the A and C directions. If you keep each individual axis separate, it's easy to predict, but trying to intentionally mix axes is very unpredictable.

All the best of luck to you, though :)


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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 7:43 pm 
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Thanks for your encouragement Arya. I've managed to perpetrate a shallowish cushion rectangle design - part of a series based on John Wren's Robin design - with very nice peroformance that I'm hoping to use. So far I've cut it in Merelani mint, and it's truly lovely. I'll try and post some pics soon, but my skills are minimal at the moment. Hold your breath!
How can I include a jpeg image in a post? I've tried without success to include a GemRay pic - error message says "extension is not allowed".


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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 8:06 pm 
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Here's a quick tutorial on how to host your images on Imgur and post them:
viewtopic.php?p=211439#p211439
Alternatively, you can also use imageshack to host:
viewtopic.php?f=17&t=3679

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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 8:57 pm 
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Thanks for the tutorial link. Very straightforward. All went well at Imgur until I tried to copy the image link. It generated an octet stream of 67000 characters instead of a link, and there didn't seem to be any solution. What am I doing wrong? Sorry to be a nuisance. This is what got pasted (abbreviated)

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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 9:30 pm 
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That's strange--looks like some Javascript. It should also give you a link on the > side of the page. If you just c/p that it should work.

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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 9:56 pm 
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Thanks. That seems to resolve the problem. Now the text format has altered, so text lines are not wrapped and are many screen widths long.
Any suggestions? I'm not having a good day online.


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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 10:22 pm 
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Or, send me the pic, I'll get it posted.


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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 11:00 pm 
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Thanks Barbra. I'll try posting the pic. This is just a GemRay jpeg for a 1.33 L/W Barion cushion rectangle.
Image

If the sapphire looks like this I'll be quite happy.
Edit:
Hmm. Well, the image is accessible now, but it's still not appearing directly. There must be a way of embedding the image in the post. I just don't know how. Yet.


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 Post subject: Re: Sapphire colour problem.
PostPosted: Wed Oct 07, 2015 11:17 pm 
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If I remember right, you can click on the picture once it is uploaded and get it to display, and then copy the URL.


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