Post subject: Determining garnet variety: pale pastel color-change pyrope?
Posted: Sun Jan 27, 2019 4:43 pm
Valued Contributor
Joined: Wed Jul 22, 2015 6:39 am Posts: 166
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
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 ?
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.
Post subject: Re: Trying to determine garnet variety
Posted: Sun Jan 27, 2019 6:57 pm
Gold Member
Joined: Thu May 12, 2016 2:18 am Posts: 1542
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.
Post subject: Re: Trying to determine garnet variety
Posted: Sun Jan 27, 2019 8:37 pm
Site Admin
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
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)
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.
Post subject: Re: Trying to determine garnet variety
Posted: Mon Jan 28, 2019 7:15 am
Valued Contributor
Joined: Wed Jul 22, 2015 6:39 am Posts: 166
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".
Post subject: Re: Trying to determine garnet variety
Posted: Mon Jan 28, 2019 3:32 pm
Platinum Member
Joined: Fri Feb 24, 2006 1:20 am Posts: 2756 Location: Southern California, U.S.A.
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.
Post subject: Re: Trying to determine garnet variety
Posted: Mon Jan 28, 2019 4:18 pm
Platinum Member
Joined: Fri Feb 24, 2006 1:20 am Posts: 2756 Location: Southern California, U.S.A.
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.
Post subject: Re: Trying to determine garnet variety
Posted: Mon Jan 28, 2019 6:18 pm
Valued Contributor
Joined: Wed Jul 22, 2015 6:39 am Posts: 166
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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