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 Post subject: Spinel and tilt window
PostPosted: Thu Sep 03, 2020 7:30 am 
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Hello world,

I recently feel in love with spinel instead of sapphire. However, all spinels I have seen so far have a serious tilt window worse than the average sapphire. From what I have read online, it was due to the refractive index of spinel is lower than sapphire's refractive index.

I wonder if certain precision cut spinel may have smaller tilt window. Thank you


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 Post subject: Re: Spinel and tilt window
PostPosted: Thu Sep 03, 2020 2:49 pm 
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Yes if you have the same exact cut, proportions on each of your two samples, the tilt performance is most likely going to be different. You can run the designs through software to compare this performance.

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 Post subject: Re: Spinel and tilt window
PostPosted: Fri Sep 04, 2020 1:36 am 
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The RI of sapphire and spinel is so close I doubt you would notice much difference. I think what you are seeing is not due to the refractive index but the cutting angles used in the particular stones you looked at.

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 Post subject: Re: Spinel and tilt window
PostPosted: Sat Sep 05, 2020 4:45 am 
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Well, I don't have any gemstone software

Precision Gem wrote:
The RI of sapphire and spinel is so close I doubt you would notice much difference. I think what you are seeing is not due to the refractive index but the cutting angles used in the particular stones you looked at.


What do you mean it is due to the cutting angles? Actually, Gene I saw one of your beautiful work posted on a gemstone forum, and it has the same tilt window problem as most spinel I have seen. I owned two sapphires precision cut by you, and they did not have this huge tilt window problem.

Attachment:
tilt window.png
tilt window.png [ 403.84 KiB | Viewed 574 times ]


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 Post subject: Re: Spinel and tilt window
PostPosted: Sun Sep 06, 2020 10:35 pm 
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Here are plots of the performance for the cushion you referenced in your photo that I cut.

The first one is rendered with a sapphire, the second with Spinel. They are almost identical.
Attachment:
Sapphire.png
Sapphire.png [ 26.14 KiB | Viewed 559 times ]

Attachment:
Spinel.png
Spinel.png [ 26.68 KiB | Viewed 559 times ]

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 Post subject: Re: Spinel and tilt window
PostPosted: Sun Sep 06, 2020 10:37 pm 
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Animated render of each material.
Attachment:
SapphireAnamation.gif
SapphireAnamation.gif [ 2.79 MiB | Viewed 559 times ]


Attachment:
SpinelAnamation.gif
SpinelAnamation.gif [ 2.73 MiB | Viewed 559 times ]

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 Post subject: Re: Spinel and tilt window
PostPosted: Mon Sep 07, 2020 2:44 pm 
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Maybe a 30 degree tilt instead of a 10 degree will reflect the image better. Also software uses absolute numbers to calculate the results, the actual stone is cut on machine by a person, so as precise as you say it is cut, it is only as precise as the machine and the cutter. Not the software. JMHO.....

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 Post subject: Re: Spinel and tilt window
PostPosted: Wed Sep 09, 2020 1:24 pm 
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glhays wrote:
Maybe a 30 degree tilt instead of a 10 degree will reflect the image better. Also software uses absolute numbers to calculate the results, the actual stone is cut on machine by a person, so as precise as you say it is cut, it is only as precise as the machine and the cutter. Not the software. JMHO.....


Wasn't it your idea to run the design through software to see the difference?

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 Post subject: Re: Spinel and tilt window
PostPosted: Wed Sep 09, 2020 2:47 pm 
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Just for fun set the RI of the material to Quartz 1.545 without changing anything else in the cutting diagram.. I would like to see that.


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 Post subject: Re: Spinel and tilt window
PostPosted: Wed Sep 09, 2020 3:08 pm 
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Precision Gem wrote:
glhays wrote:
Maybe a 30 degree tilt instead of a 10 degree will reflect the image better. Also software uses absolute numbers to calculate the results, the actual stone is cut on machine by a person, so as precise as you say it is cut, it is only as precise as the machine and the cutter. Not the software. JMHO.....


Wasn't it your idea to run the design through software to see the difference?



Yes. And I also said " the tilt performance is most likely going to be different" which it is slightly and the falloff is more drastic in Spinel.


My comment about a 30 degree tilt is based solely on the image the poster posted, which looks to me like the two stones in the image are angled more than 30 degrees. A majority of gemstones tilted past 10 degrees are prone to a dramatic visible window and performance drop.

What is the drop off between the two when the tilt is higher is the thought I had and again based only on the posters post of why the big difference. Yes I can run the performance tests myself between the two species and have but we both know that the design and angles will dictate the results, and it appears you have the actual design to test.

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 Post subject: Re: Spinel and tilt window
PostPosted: Wed Sep 09, 2020 8:55 pm 
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Here you go in Quartz. Not a design I would cut in quartz.

For me, any designs I cut in Sapphire, I would also cut with no changes in Spinel. From the plots there is essentially no difference. If I didn't identify them, you would never know which is which.

Attachment:
Quarzt_Cushion.png
Quarzt_Cushion.png [ 27 KiB | Viewed 399 times ]


Attachment:
Quartz_Cushion.gif
Quartz_Cushion.gif [ 2.92 MiB | Viewed 399 times ]


Attachments:
Quartz_Cushion.gif
Quartz_Cushion.gif [ 2.92 MiB | Viewed 399 times ]

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 Post subject: Re: Spinel and tilt window
PostPosted: Wed Sep 09, 2020 9:23 pm 
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Thanks Gene. Definitely a big difference in performance on the tilt angle in quartz.

What are the pavillion angles being used?

Also a big difference in dispersion being shown.


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