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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sun Mar 25, 2018 8:01 pm 
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Jim,
Glad you like it. It is an unusual color combo for sure. This is one of the rare ones where both colors would be an attractive stone on its own. The change is near 100%. I have never seen another quite like it. Lucky for me it was in a mixed lot of stones that was offered to me, and of course I picked it out immediately. I will never sell it, so I don't mind talking about the price. The whole stone was well under $100.00.

Steve


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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Mon Mar 26, 2018 5:57 am 
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Aaaaaand Steve scores!

I wouldn't want to sell it either but I'm poor so I probably would but it would take a tall stack of
benjamins to do so especially after it was cut.

Congratulations on the buy.

Jim


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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Wed Apr 11, 2018 1:05 am 
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Location: Vancouver, Canada
Thanks to Barbra for running the sample stones I sent her through a GemmoRaman and getting these results.

(BTW - the carat weights don't match perfectly with mine posted previously as I polished windows into the stones to get RI and assist with the analysis)
4.12ct color change pyralspite
2.28ct. Spessartite: not orange but the Raman can not see color
6.74ct. color change pyralspite
5.74ct. Color change pyralspite

I'm intrigued as I don't understand how they can show apparent dichroism despite being garnets.

Pyralspite Colour-Change Garnet Scan:
Image

Green/Reddish Spessartite Garnet Scan:
Image

I also sent along 3 other garnets from what appeared to be different families within the larger parcel. The scan below is for a small garnet that's violet/purple in daylight and brighter red-purple in incandescent. It turned out to be a Malaya garnet:
Image

I'm in the process of cutting a garnet from the Pyralspite set, which shows a greyish-purple in CFL and startlingly bright bubblegum pink under 40W incandescent. I'll post pics once I get it finished.

Thanks Barbra for the analysis!

-Allan

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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Wed Apr 11, 2018 1:54 am 
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Dang, I need a GemmoRaman. Serious equipment envy :?


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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Wed Apr 11, 2018 10:43 am 
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I will try to address your other concerns today, Allan.
I still have your samples. May not have an opportunity to get to the Post Office until I return from Sinkankas.


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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Wed Apr 11, 2018 1:46 pm 
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Hi Barbra - no rush on returning the samples. If you want to hold on to them for a bit more investigation no problem, I have others I'm playing with :)

-Allan

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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Wed Apr 11, 2018 8:27 pm 
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Hi Allan, I'm back.
Just tested your stones with the polariscope. They do not "blink", as a doubly refractive stone would, they have irregular blinking which appears as snake-like wobbling bands or crosshatching.
Typical of anomalous double refraction (ADR) seen often in garnets, red spinel and diamonds. It indicates internal strain.
Image
To verify, I orientated the stones in their lightest position and then quickly turned the analyzer 90°.
The stones appeared noticeably lighter. SLAM DUNK: The stones are singly refractive and are exhibiting ADR.

Then, I pulled out my calcite dichroscope and examined all three stones. None were dichroic.
Perhaps their color changing character is causing the illusion of dichroism, but alas, they are not dichroic.

Are they exhibiting the Usambara Effect?
There is a lot of misinformation circling around the Usambara effect. It is NOT the alexandrite effect and has nothing to do with color change as a result of viewing with different light temperatures.

The phenomenon was originally observed in Nchongo chrome tourmalines.
Elise Skalwald wrote:
The Nchongo chrome tourmalines are a verdant green in both transmitted and reflected light. Once the thickness of the mineral reaches a critical point, dependant on concentration of the elements chromium and vanadium, the perceived color of the transmitted light suddenly shifts to red (see transmission curves). This occurs viewing either the ordinary or the extraordinary ray at about the same thickness.

In short, the thickness of the sample is responsible for a change of color when viewed with transmitted light.
So is our parcel of famous garnets exhibiting the Usambara Effect?
Nope.


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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Thu Apr 12, 2018 1:21 am 
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I'd note that the two effects are sort of related, in that chromium (and vanadium(??)) is involved in producing both, most notably in alexandrite as well as chrome tourmaline, presumably because of their absorption features in both red and green. I got some of the blue spessartine garnets from NewEra a few months back which are loaded with vanadium and some chromium. Some do appear to show a rather pronounced usumbara effect in addition to a color shift or change (some show a minor change while others shift more). I haven't examined it more closely though.


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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Thu Apr 12, 2018 10:33 am 
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That's possible, Stephen.
The point is that the effects are the result of 2 different reactions to light.
Usambara: transmitted (not reflected) light with one temperature
Color Change: viewed with light of 2 light temperatures, one very warm, one very cool.

The specimens found in the type locality of the Usambara tourmaline were not color changing.

Would you be kind enough to post pictures of your spessartite.

Sidebar: when I was a student in Germany I was taught that trace amounts of V were essential in color change material.
chromium schromium :D


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