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 Post subject: Unusual Colour-Change Stones
PostPosted: Sat Mar 24, 2018 3:11 am 
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Hi everyone - got a puzzler here which I would appreciate some help with.

I recently got a parcel of colour-change garnets that go from greenish/bluish in 5000k light to pink/red in 3000k. These are some of the odder ones that got me wondering if they are garnet after all.

They look unusual when rotated - the colour intensity changes quite suddenly rather than gradually - and appear to be dichroic, though looking through my dichroscope I do not see obvious dichroism. I'm now questioning my eyes and sanity.

Stone ct SG RI Magnetism
A - 2.30ct - SG=4.18 - RI=>1.81 - magnet picks up
B - 4.17ct - SG=3.97 - RI=1.76 - magnet drags
C - 5.81ct - SG=3.95 - RI=1.77 - magnet drags
D - 6.78ct - SG=3.92 - RI=1.76 - magnet drags

My eyes and refractometer skills aren't good enough to get any better numbers (I was cursing most of the time trying to see the damned shadow lines!). And stone A nearly drove me mad until I realized I had a stone over the RI ability of the refractometer :roll:

So, from the info I've got, and assuming that they are not dichroic - garnet seems likely. Any other ideas? Any other tests recommended? I can post phone videos also, but I have no idea how to resize them from 1080p to something more reasonable...

Thanks!

-Allan

Pics of the stones in various orientations (5000k light) below:
Stone A:
Image
Image
Image
Image

Stone B:
Image
Image
Image
Image

Stone C:
Image
Image
Image
Image

Stone D:
Image
Image
Image
Image

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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sat Mar 24, 2018 5:29 am 
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Some CCG show an Usambara effect if large enough..

Your item is large enough imo.

Attachment:
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IMG_1770.jpg [ 38.59 KiB | Viewed 2012 times ]


If you cut them you need a long ( long oval, emerald or marquise) shape. If the stone is to short the effect get lost.

This is a green tourmaline with UE

Attachment:
IMG_1768.jpg
IMG_1768.jpg [ 70.39 KiB | Viewed 2012 times ]


Attachment:
IMG_667.jpg
IMG_667.jpg [ 44.89 KiB | Viewed 2012 times ]

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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sat Mar 24, 2018 11:30 am 
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This looks very similar to the spessartite beads I recently purchased at a show.
They are from Tanzania.
With the polariscope: ADR.
Magnetic attraction: some get picked up, others drag.


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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sat Mar 24, 2018 7:37 pm 
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Thanks for the feedback! I never thought garnets would show umbasara effect, but now I've learned a few things! One of which is how much I dislike using a refractometer :)

On the polariscope it shows blue to red change depending on the polaroid positions. Interestingly, the thicker areas always show red - lending strong support to the umbasara effect argument.

Double-checking with the dichroscope there is no dichroism that i can see...

So... I think the verdict is cc garnets (some go blue to red, some green to red, some purple to red...) with umbasara effect messing with me.

They are all from Tanzania, so possibly could be from the same find as the beads Barbra posted earlier.

Thanks everyone, I'm feeling a bit better that my gem id skills are not a complete wash :D

-Allan

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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sat Mar 24, 2018 10:05 pm 
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I'm not sure it is the Usambara effect.


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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sat Mar 24, 2018 10:25 pm 
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I’ll try to figure out how to post videos of the stones. The videos show how quickly it switches or intensifies to red, faster than I’d expect just from rotating the stone.

-Allan

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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sun Mar 25, 2018 1:59 am 
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https://gem-a.com/images/Documents/JoG/ ... 5---LR.pdf

page 476 and 477

Here a green tourmaline.

I have seen UE CCG in real life - a friend of mine bought rough stones and some turned red in thicker areas.

Maybe Allen can send some item to Barbra?

Barbra, can you stick 3-4 beads on an adhesive tape in a row - maybe they show the UE...
-------

UE in synthetic alexandrite

https://www.gia.edu/gems-gemology/sprin ... e-sundberg

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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sun Mar 25, 2018 2:34 am 
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Some more clues, but I'm still not sure what to make of this all. Stone A is clearly different from the other stones B, C, and D which are all very similar in colour and properties.

Sorry, my videos are all from my iphone in .MOV format and are uploaded to my personal server. Most people will have to right-click the link and download the file, then use VLC or something like that to play the file.

Polariscope video - stone A, B, and C
http://www.omnifaceter.net/ccgarnet/polariscope.MOV

Video of stone A: (updated to correct stone!)
http://www.omnifaceter.net/ccgarnet/stoneA.MOV

Video of stone B: (updated to correct stone!)
http://www.omnifaceter.net/ccgarnet/stoneB.MOV

Video of stone C: (updated to correct stone!)
http://www.omnifaceter.net/ccgarnet/stoneC.MOV

Imay have to poke around with a few more of them. I think there are 35 stones that fall into 4 broadly similar colour and colour change groupings.

-Allan

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Last edited by gembug on Sun Mar 25, 2018 12:34 pm, edited 2 times in total.

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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sun Mar 25, 2018 3:36 am 
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I am a bit confused with the first video....

Maybe sapphire ( your RIs)???

Second video I can't view.

Last looks like UE but if not garnet maybe dicroism/pleochroism...

But RI, magnetic reaction????

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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sun Mar 25, 2018 10:02 am 
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Have you checked Spectra yet?


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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sun Mar 25, 2018 11:00 am 
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Alan, if you want to send me a sample, I'd be happy to test it on the GemmoRaman.
I'll even send back the sample. :wink:


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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sun Mar 25, 2018 12:21 pm 
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I've re-uploaded stone A - something was wrong with the file.

Here's the stone specifics again:
Stone ct SG RI Magnetism
A - 2.30ct - SG=4.18 - RI=>1.81 - magnet picks up
B - 4.17ct - SG=3.97 - RI=1.76 - magnet drags
C - 5.81ct - SG=3.95 - RI=1.77 - magnet drags
D - 6.78ct - SG=3.92 - RI=1.76 - magnet drags
None of the stones display any dichroism through a dichroscope.

I don't have a spectroscope so can't check the spectrum... maybe next purchase!

Barbra - I'd appreciate if you'd run it through the Gemmo... please PM me where to send and I"ll send you some samples.

They definitely have me baffled!

Thanks everyone

-Allan

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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sun Mar 25, 2018 1:02 pm 
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Stone A and C show UE in my opinion.

B not...

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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sun Mar 25, 2018 4:39 pm 
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Lots of unique and different color change garnets coming out of East Africa these days. Here is a 16 carat one I picked up on my last trip.


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 Post subject: Re: Unusual Colour-Change Stones
PostPosted: Sun Mar 25, 2018 7:26 pm 
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I don't know much at all about color change gems but that is really beautiful.
the color is phenomenal under both types of light and the change looks to be complete.
If you don't mind me asking how much was that gem or did you literally pick it up off the ground?

Thanks, Jim


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