January 24 Through February 4—TUCSON, ARIZONA: Annual show
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 Post subject: Determining garnet variety: pale pastel color-change pyrope?
PostPosted: Sun Jan 27, 2019 4:43 pm 
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Hello

I picked up a small parcel of odd colour change garnets on my recent travels.

I brought them in rough form along with a load of garnets and had them cut locally whilst in the sticks. I brought them as they appeared quite colourless in rough form.

I was hoping to gain some insight as to what variety of garnet they maybe.

Ok, so pics are taken in daylight, incandescent, and under SW UV

Image

Image

they do have a noticeable but weak magnetism with an earth magnet

Am I right in saying the reaction to UV is down to chromium ?

At first I was thinking grossular garnet but they are not entirely colourless and do have a slight purplish tinge. Also am I correct in saying grossular garnets have weak to no magnetism ?

Also, no reaction to LW UV.

Many thanks for any insight.


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 Post subject: Re: Trying to determine garnet variety
PostPosted: Sun Jan 27, 2019 5:45 pm 
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Very odd. The only garnets I know of that have any fluorescence are some tsavorites/mint garnets, which also glow orange. I don't think chromium is responsible. I would gueds grossular with a pinch of vanadium and some other stuff, but sg and ri would be very useful.

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 Post subject: Re: Trying to determine garnet variety
PostPosted: Sun Jan 27, 2019 6:57 pm 
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Not saying these are, but there is a lot of color change glass with a broad range of shift colors out there these days. Not sure about fluorescence in the glass having never tested that. The glass is formed and tumbled into very convincing rough and sold throughout the bush and in fact most markets in the World these days.


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 Post subject: Re: Trying to determine garnet variety
PostPosted: Sun Jan 27, 2019 8:37 pm 
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Grossular would be the only variety of garnet that actually occurs colorless....
In the colorless state it's composition would be pretty close to Ca3Al2Si3O12

The usual substitutions for the Aluminum are iron (Fe3+), manganese and chromium.
None of those elements would not cause a color change. (unless the chromium miraculously paired up with some vanadium) :smt017

Where are these from?
How about LWUV reaction?


Therefore, I think the question might be "Trying to determine what these are"?
Glass is definitely a possibility.


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 Post subject: Re: Trying to determine garnet variety
PostPosted: Mon Jan 28, 2019 12:39 am 
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It wouldn't be too miraculous to get some Cr and some V both. They're geochemical buddies and both happily hop into grossular.

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 Post subject: Re: Trying to determine garnet variety
PostPosted: Mon Jan 28, 2019 7:15 am 
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thanks everyone for the detailed answers, much appreciated.

No reaction to LW UV for sure.

I also suspected glass but they do have natural looking inclusions, one is full of loupe visible needles which we've all seen in garnet.

The rough had the usual pitting I get in a lot of garnet rough.

Also unsure if glass would have magnetism.

Ive had pink malaya garnets previously (still have a few somewhere in the depths of my safe) which have flouresced under SW UV, I was under the impression it was the chromium causing this, however I am not entirely sure what put this idea in my head lol

I also came across a GIA article about chromium gcolour change grossular garnets from Ethiopia, but the colours are very different to these ones.

Origin is unclear, although I brought them in Tanzania and I was told they were locally mined Ive found theres so much material going through the place that people just say all the material on the market is "local".


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 Post subject: Re: Trying to determine garnet variety
PostPosted: Mon Jan 28, 2019 12:03 pm 
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Some of the glasses are magnetic now, and I doubt inducing pitting would be a problem. Needles ought to be a good indicator though.

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 Post subject: Re: Trying to determine garnet variety
PostPosted: Mon Jan 28, 2019 12:39 pm 
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Thanks Stephen
I'll leave these aside, they'll probably sink to the depths of my gem pile lol


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 Post subject: Re: Trying to determine garnet variety
PostPosted: Mon Jan 28, 2019 3:32 pm 
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Raj_uk wrote:
Thanks Stephen
I'll leave these aside, they'll probably sink to the depths of my gem pile lol

Can you point me to the G & G article about CC grossular garnets from Ethiopia? Things can change fast with new discoveries in this field but in the past the only major CC garnets were all pyrope-spessartines. I've read about treated grossulars from Brazil that showed CC but none were colorless. The thing that intrigues me about your stones are the natural, presumably rutile, inclusions.

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 Post subject: Re: Trying to determine garnet variety
PostPosted: Mon Jan 28, 2019 3:45 pm 
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of course, here we go -

https://www.gia.edu/gems-gemology/summer-2018-gemnews-discovery-of-color-change-chrome-grossular-garnets-from-ethiopia

I had to look twice at the colour change pictures in the article, its quite a subtle change


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 Post subject: Re: Trying to determine garnet variety
PostPosted: Mon Jan 28, 2019 3:48 pm 
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all of the garnets in the parcel have very typical natural looking inclusions, if they didn't Id be happy enough to resolve them as glass.


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 Post subject: Re: Trying to determine garnet variety
PostPosted: Mon Jan 28, 2019 4:18 pm 
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Raj_uk wrote:
all of the garnets in the parcel have very typical natural looking inclusions, if they didn't Id be happy enough to resolve them as glass.

Thanks for the link. The Sri Lankan discovery was new to me as well. Garnets are ever-fascinating to me but I haven't been keeping up with the new finds.

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 Post subject: Re: Trying to determine garnet variety
PostPosted: Mon Jan 28, 2019 4:36 pm 
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If you are going to Tucson, we could do a Raman for you.


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 Post subject: Re: Trying to determine garnet variety
PostPosted: Mon Jan 28, 2019 5:07 pm 
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Now we're talking. =D> =D> =D>


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 Post subject: Re: Trying to determine garnet variety
PostPosted: Mon Jan 28, 2019 6:18 pm 
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dchallener wrote:
If you are going to Tucson, we could do a Raman for you.


aw that's a really kind offer : ) Thank you so much.

Man, Ive always wanted to go.. just the distance and expense.

Im off to Thailand next week, my wife lives there and is having an operation so Ill be there for some time nursing her back to health.

Maybe next year Ill get to go to Tuscon. I always drool looking at the pictures of piles of rough GO members have acquired.
And it would be really nice to meet the people there, theres not many gem addicts on this side of the pond.


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