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 Post subject: Advice orienting strongly dichroic sapphire
PostPosted: Wed Feb 15, 2023 12:27 am 
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I have a fine piece of sapphire rough in a rectangular prism shape. Along the c axis the colour is a lovely (quite) dark purple. Along the length of the rectangle the colour is a light purple, and across the width it is colourless. I would like to avoid placing the table exactly on a crystal plane, and will improve the yield considerably if the stone is tipped around the light purple axis. I would also like to lighten the colour to show the purple without washing it out. If the stone were cut with the table perpendicular to the c axis it would probably be almost black. The big question is how much to tilt. The best yield would need a 45 degree tilt, but this would likely wash out the purple. My suspicion is somewhere in the 25-30 degree range, but that's just a plausible guess. My concern is to get the best possible colour without sacrificing the finished weight, but to do this need first to find the optimum colour. I had thought, for example, of experimenting with a similar but smaller flawed piece of blue sapphire with similar pleochromism. Any suggestions about how to proceed would be gratefully received. I'll post some pics when I get a round tuit.


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 Post subject: Advice orienting strongly dichroic sapphire
PostPosted: Wed Feb 15, 2023 10:56 am 
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Becauze of its hexagonal crystal structure sapphire can only be dichroic, not trichroic. That is also an odd combination of colors for sapphire--is there any chance it might be iolite?

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 Post subject: Advice orienting strongly dichroic sapphire
PostPosted: Wed Feb 15, 2023 2:14 pm 
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It would also help if you provided its clarity and the style of cut you propose for it. I'm assuming based on your statements of best wait maybe a mixed or step cut. Pics would be nice. I am with Stephen, your description is more like Iolite or even Color banding in Amethyst. Is there enough testing that positively identified it as corundum.

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 Post subject: Advice orienting strongly dichroic sapphire
PostPosted: Thu Feb 16, 2023 2:21 am 
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That was fast! Thanks for your comments gentlemen. I bought the stone from a Tanzanian dealer years ago, and paid for sapphire at the time. I think the slight purple along the long axis of the stone may have just been confusion on my part, because the viewing direction was awkward. I haven't yet cut any windows, but the stone's shape is definitely consistent with sapphire. I'll check this out when I can find my refractometer. I do have three pieces, all of this colour. One has a clear hexagon pattern in the heart of the stone. I think they may have come from an estate sale. I'll do my best to get some useful pictures, but it will take a day or two. For now let me correct the colours to deep purple, near colourless and near colourless along the three axes of the rectangular prism.
Regarding the intended cut - these are early days and I don't plan to rush - probably a modified mixed cut cushion. It really depends on the degree of tilt. The rough is ten carats or so, most likely unheated, and with no major internal problems apparent at the moment. A nice piece.
Later: I was able to find the refractometer, and for the one stone with a polished window the RI is 1.763. I'm fairly confident all three are corundum.


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 Post subject: Re:Advice orienting strongly dichroic sapphire
PostPosted: Thu Feb 16, 2023 2:33 pm 
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Post a picture please.


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 Post subject: Re: Advice orienting strongly dichroic sapphire
PostPosted: Thu Feb 16, 2023 3:01 pm 
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Ok then, tilt it 45 degrees to get the highest yields or tilt it for color. That's an easy one.
Truthfully I would probably choose color with best tilt angle for yield. A beautiful colored purple sapphire cut well will make up for some loss in weight.
Would love to see some pics.

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 Post subject: Re: Advice orienting strongly dichroic sapphire
PostPosted: Thu Feb 16, 2023 11:30 pm 
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Obtaining realistic images has not been easy. I ended up cutting a small rectangular window in card, then using a very powerful flashlight to transmit light through the stone. The images have been edited to give a more realistic sense of the colour, but still leave much to be desired. Just a few comments:

Image 1 shows the view from above looking down the purple c axis. Under iso lighting this would appear almost black.

Image 2 shows the view across the narrow dimension of the rectangle of image 1. The left vertical edge corresponds to image 1. The two natural faces perpendicular to the c axis are parallel and relatively flat, as is evident in this image (the two vertical edges).
The four remaining faces are irregular and don't photograph well. The light areas are representative of colour seen from a loupe-type view. There is probably some internal reflection of the transmitted light causing the slight purple tint - hence my confusion about three colours.

Image 3 shows the view along the long dimension of the rectangle. The same comment about colour tint applies here too.

With regard to the advice about orientation - I agree entirely. My dilemma is in determining this "best" colour, and the related tilt angle, without creating a stone either too light or too dark - sort of a Goldilocks situation. Any ideas or suggestions about how this might be done are most welcome. As I mentioned at the outset, I do have a smallish piece of blue sapphire with very similar pleochromism on which I can experiment.

My thanks to everyone who has replied btw.


Attachments:
3 purple sapphire perp to c axis along long x axis.jpg
3 purple sapphire perp to c axis along long x axis.jpg [ 211.46 KiB | Viewed 3108 times ]
2 purple sapphire perp to c axis along short y axis.jpg
2 purple sapphire perp to c axis along short y axis.jpg [ 240.37 KiB | Viewed 3108 times ]
1 purple sapphire along c axis.jpg
1 purple sapphire along c axis.jpg [ 199.7 KiB | Viewed 3108 times ]
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 Post subject: Re: Advice orienting strongly dichroic sapphire
PostPosted: Fri Feb 17, 2023 1:38 am 
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Think you can just take a couple of photos laying on a sheet of white paper in a couple of different directions.

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 Post subject: Re: Advice orienting strongly dichroic sapphire
PostPosted: Fri Feb 17, 2023 3:13 am 
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I can certainly do that, but I'm not sure it would show much. I tried placing the stone on white paper, but it just looks black whichever way it's turned. There's slightly more colour when it's in oil, but not much. The problem at the moment is that none of the faces are either flat or polished, and apart from the natural crystal faces the remaining four are quite rugged. I'm reluctant to take the plunge and preform the stone even as a polished rectangular prism. What do you think? Should I do this? What would be gained?
I have a GIA stereo microscope with an immersion cell and dark field lighting, but unfortunately no camera. I could perhaps improvise something though if it would be useful.


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 Post subject: Re:Advice orienting strongly dichroic sapphire
PostPosted: Fri Feb 17, 2023 2:08 pm 
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Ok, just wanted to observe it generally. Does not help completely just seeing it internally. You have already stated there are polished surfaces. I refer back to my previous suggestion of orientation. Good luck and would like to see the end result.

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 Post subject: Re: Advice orienting strongly dichroic sapphire
PostPosted: Fri Feb 17, 2023 3:43 pm 
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salik wrote:
I can certainly do that, but I'm not sure it would show much. I tried placing the stone on white paper, but it just looks black whichever way it's turned. There's slightly more colour when it's in oil, but not much. The problem at the moment is that none of the faces are either flat or polished, and apart from the natural crystal faces the remaining four are quite rugged. I'm reluctant to take the plunge and preform the stone even as a polished rectangular prism. What do you think? Should I do this? What would be gained?
I have a GIA stereo microscope with an immersion cell and dark field lighting, but unfortunately no camera. I could perhaps improvise something though if it would be useful.



You understand that how it looks on white paper is the best indicator how it will look after cutting. That is why Gregg asked to see it that way. Unless it has a opaque skin that comes off it is likely to be almost black after cutting as well.


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 Post subject: Re:Advice orienting strongly dichroic sapphire
PostPosted: Fri Feb 17, 2023 10:33 pm 
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Of course I understand about the white paper test. My point is that the test is not really fair with the rough in its present state. Once the rough has been polished yes, the test will be useful. For now I've attached photos with the rough in its present state, and also images taken in oil immersion with dark field illumination. The images are corrected for 3600K light, using a ground glass screen or oil immersion, both with the dark field illumination. The camera angle is about 20 degrees to the vertical - the closest I could get. Through the microscope the darks are darker, and the lights lighter as a consequence. The colour difference is significant, which gives me some hope that the stone can be lightened by tilting. The question, of course, is how much tilt, and then how to determine the optimum angle.
After closer examination last night, I think I will go ahead and polish flats on the four long faces. This will take me a day or two, so I ask for your patience. I'll post more pics after the polishing. The stone is not as clean as I hoped, but still fairly clean internally, especially with serendipitous orientation. The light path inside the stone with two internal reflections will be in the region of 12 to 14 mm, a not insignificant distance.


Attachments:
2316  purple saphhire from above gg screen 3600K.jpg
2316 purple saphhire from above gg screen 3600K.jpg [ 313.28 KiB | Viewed 3051 times ]
2322  purple saphhire from side gg screen 3600K.jpg
2322 purple saphhire from side gg screen 3600K.jpg [ 362.72 KiB | Viewed 3051 times ]
2326  purple saphhire  from above immersion  3600K.jpg
2326 purple saphhire from above immersion 3600K.jpg [ 658.59 KiB | Viewed 3051 times ]
2327  purple saphhire  from side immersion  3600K.jpg
2327 purple saphhire from side immersion 3600K.jpg [ 778.66 KiB | Viewed 3051 times ]
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 Post subject: Re: Advice orienting strongly dichroic sapphire
PostPosted: Sat Feb 18, 2023 10:42 am 
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So, a caveat. One of the names by which iolite goes is "water sapphire". Sometimes--particularly in East Africa--you will get a miner or dealer selling iolite as "sapphire". There is not really an intent to deceive, I don't think. Rather, they simply don't understand the difference or they don't understand what a big difference the modifier "water" makes in this case.


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 Post subject: Re: Advice orienting strongly dichroic sapphire
PostPosted: Sat Feb 18, 2023 5:15 pm 
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Truthfully I was just looking for a general picture of the rough, like looking at it in the hand or between the fingers with a white background. I don't know about black in this stone based on these images, maybe an overall saturated purple. Many here as well as I would say that depends on the cut and how much light your cut lets in and returns back. The conundrum we all face, as with you dealing with the decision of best orientation based on your original post, does size matter or a beautiful stone. I am not quite convinced it is corundum, but I take your word for it. With a lot of rough placing the color in the pavilion is the way to go.
With the pics you have posted it is really hard to conceive your 45 degree tilt position. Another question, are these images of just one stone?

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 Post subject: Re: Advice orienting strongly dichroic sapphire
PostPosted: Sat Feb 18, 2023 7:51 pm 
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Thanks for your comments. Yes, this is just the one stone. Once I get the rough polished I'll determine the RI to settle the corundum question. I agree about the 45 degree tilt. I'm not going to be greedy at the expense of colour.


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