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 Post subject: Seperating Natural from Heated Citrine
PostPosted: Thu Feb 14, 2019 3:10 pm 
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When I was in Tucson at the AGA meeting, I spoke with Dr. Claudio Milisenda who has been with the German Gemmological Association (DGemG) and it considered an expert on quartz.

Naturally occurring citrine is dichroic.
Heated amethyst is not.


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 Post subject: Re: Seperating Natural from Heated Citrine
PostPosted: Thu Feb 14, 2019 6:05 pm 
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I think it was on this forum that somebody once gave the reference of this excellent article from 2013 about the colors of quartz. I have printed it and sometimes re-read it with delight, it is so very clear.

http://ctminsoc.org.za/articles/the-colours-of-quartz

If a citrine is natural, it is Al-bearing, and it is pleochroic.
If it is issued from a Fe-bearing amethyst heat treated, it will not show pleochroism.


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 Post subject: Re: Seperating Natural from Heated Citrine
PostPosted: Thu Feb 14, 2019 11:14 pm 
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I like the phrasing on that, Isi, though I am willing to believe there could be natural occurrences of 'heated amethyst' type citrine somewhere (not that it would ever be easy to prove it!). Of course pleochroism likely won't help with radiation-induced colors since those are also li/al based.

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 Post subject: Re: Seperating Natural from Heated Citrine
PostPosted: Fri Feb 15, 2019 12:26 pm 
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Perhaps there may be naturally occurring heated amethyst....do we need to concern ourselves with it?

If one wishes to buy or sell unheated citrine i would suggest sticking to material which can be separated with pleochroism.

The other possibilities (or probabilities) are subjects for mineralogical doctoral projects...or post doctoral projects, (Right up the Challener's street by the way), but perhaps not something the average citrine buying gemologist should be too concerned about.

Thoughts?


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 Post subject: Re: Seperating Natural from Heated Citrine
PostPosted: Fri Feb 15, 2019 6:37 pm 
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I'd say if you want untreated citrine you need to buy uncut rough close to the source and have it cut yourself. Zambian rough can be bought this way easily, with a crystal form that is sufficiently distinctive to exclude it being typical amethyst starting material for heating.
I don't think irradiated citrine is very obscure though. Aside from lemon quartz which is irradiated, pleochroic and ubiquitous there are other less obvious hues that can be produced that way, many of which the late Liccini mentioned on his webpage. So I would not take pleochroism alone as an effective defense against treatment, just for separating 'heated amethyst' style citrine from al/li citrine or however you prefer to call it.
Aside from that, in my opinion the broadest and most interesting color range of citrines are treated one way or the other, so if you like citrine you should just accept that.

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 Post subject: Re: Seperating Natural from Heated Citrine
PostPosted: Fri Feb 15, 2019 7:50 pm 
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Stephen wrote:
in my opinion the broadest and most interesting color range of citrines are treated one way or the other, so if you like citrine you should just accept that.


I agree, but over the years we routinely get queries on identifying unheated citrine, so I thought it would be a good idea to post my new information. :D


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 Post subject: Re: Seperating Natural from Heated Citrine
PostPosted: Wed May 29, 2019 12:42 am 
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I wonder what happens when Ameythst is re-heated? Does it revert back to Citrine color?

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 Post subject: Re: Seperating Natural from Heated Citrine
PostPosted: Thu May 30, 2019 3:21 pm 
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It's the other way round...
If a citrine is re-heated, does it come back to amethyst ?
In my experience, no ; next step is : it turns milky, and then, it cracks.


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