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 Post subject: Question about irradiated brown topaz
PostPosted: Wed Dec 19, 2018 4:30 pm 
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I have questions for all topaz lovers.

Is there a way other than the fading test to determine if an orangy brown topaz has been irradiated or not ?
Is there a typical color or style of cutting for unstable brown topaz ? If there is red in the pleochroism, can it still be irradiated stuff ?

Do you think this could be irradiated and unstable ? Or is it more likely a genuine imperial topaz from Brazil ?
http://www.drouot-estimations.com/html/ ... id=9622707


Last edited by Isi on Wed Dec 19, 2018 4:38 pm, edited 1 time in total.

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 Post subject: Re: Question about irradiated brown topaz
PostPosted: Wed Dec 19, 2018 4:35 pm 
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If I saw that color and knew (somehow) that it was a real topaz, I wouldn't worry about fading.


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 Post subject: Re: Question about irradiated brown topaz
PostPosted: Wed Dec 19, 2018 5:05 pm 
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Well, the real stone is a bit more brown and a bit less pink.
Actually it is more like the middle stone on the first picture of this well known link :
http://www.palagems.com/topaz-buying-guide/


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 Post subject: Re: Question about irradiated brown topaz
PostPosted: Wed Dec 19, 2018 8:54 pm 
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Color wise, both images are more typical of precious Topaz from the Oro Preto region of Brazil that they are of unstable irradiated topaz from other locations.

Of course just seeing pictures on the Web of stones is not a reliable way to make these kinds of identifications and opinions.


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 Post subject: Re: Question about irradiated brown topaz
PostPosted: Wed Dec 19, 2018 11:19 pm 
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Isi wrote:
\
Is there a way other than the fading test to determine if an orangy brown topaz has been irradiated or not ?
Is there a typical color or style of cutting for unstable brown topaz ? If there is red in the pleochroism, can it still be irradiated stuff ?

I will try not to make my answer an unreadable wall of text but no promises.

As a short answer: irradiation status can't be determined by whether a topaz fades or not, as both natural and artificially-induced color in topaz can be either stable or unstable. We don't know why some are stable and some aren't, or exactly why some stones turn one color and some another. That said, I don't think that's the question you want answered. I think you want to make sure the stone isn't one of two other kinds of topaz:
1) non-chromium-bearing brown topaz, like the common blocky material from Pakistan's pegmatites
2) chromium-bearing topaz (ie, from the same deposits as Imperial topaz) that has been irradiated to produce a richer, unstable color.

Chromium-bearing and non-chromium-bearing topaz are two different animals. Chromium is essential to produce the 'imperial' range of colors, the pure pinks, purples, reds, oranges. Stones without chromium can be orangey-brown, red-brown, or pink-brown, but not pure orange, red or pink. Stones with chromium can also have a brownish component but even then I think they're noticeably richer in color. I think they could be distinguished because chromium-bearing stones will have a saturated pink component to the pleochroism that isn't present in non-chromium-bearing stones. You might confuse a chromium-bearing and non-chromium-bearing stone when dealing with lighter-toned stones as the difference in saturation isn't nearly so obvious, but that's unlikely to come up. Think of it like the difference between emerald and green aquamarine--not likely to mistake one for another in most circumstances. So that's possibility 1 taken care of.

That said possibility 2 exists. You can also irradiate stones that do contain chromium, like say a orangey-gold topaz from Brazil--not 'imperial' by the strictest standards but a chromium-bearing metamorphic topaz from the same deposit. These stones don't turn brown or blue like the common pegmatitic material would, but instead transform into an insanely rich orange-red, like the finest imperial topaz. This color is very unstable, and quickly fades down to a lesser stable color, usually what it started with--each stone seems to have some inherent level of color it can hold and no more. This treatment apparently caused something of a panic in the 80s, with dealers resorting to taping their stones to windows to show they were not going to fade. I wasn't there, just something I heard. Nowadays this treatment is mostly seen in stones from Katlang in Pakistan, which often contain just a bit of chromium and respond very well to this treatment, though most fade quickly down to a light orange, a champagne or light pink.

A test in sunlight is the only way to tell if a stone is photostable, but I think this treatment is uncommon in Brazilian stones nowadays.

I haven't seen many non-Brazilian topazes cut in the classic 'topaz cuts' favored for them. The non-chromian stones tend to come in blocky crystals that don't need those weird long pears and ovals. Stones from Katlang have the same crystal form as Brazilian stones, but still I haven't seen them cut that way, probably since they aren't being cut in Brazil. So the cutting style is still an indicator, though one you wouldn't want to rely on too much in case someone wants to deliberately fool you.

The first picture you linked resembles Brazilian imperial, and I would be surprised to see that purity of color in a non-chromian stone. Irradiation should be a remote possibility if it's a reliable seller.

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 Post subject: Re: Question about irradiated brown topaz
PostPosted: Thu Dec 20, 2018 1:11 am 
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Thanks to all for your opinions and especially to Stephen for your detailed answer. You quite understood my questioning. I had more or less these elements in mind but not in such a detailed way, far from it.
Yes the cutting style and pleochroism I could see made me rather confident. But I could not even see it in daylight, only under electric light... :(

Actually this stone was for sale in an auction and remained unsold.
http://www.drouot-estimations.com/html/ ... =&aff=1&r=
So I could acquire it as an after sale for approximately 2500 US dollars all fees included.
The stone is more than 30 carats so maybe even a bit too big. The catalogue picture is not quite faithful from what I saw, there is more brownish orangy and less pink in the real thing.
So I have to make a quick decision. But I know nothing about the origin or current owner apart that it is a private person who loves big stones (there were 2 huge aquamarines in the same sale).

What do you say ?
How long would it take to chromium bearing irradiated Brazilian material such as you describe before it turns back to its original color in normal daylight exposition conditions ?


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 Post subject: Re: Question about irradiated brown topaz
PostPosted: Thu Dec 20, 2018 3:46 am 
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I don't know how quickly they would fade when not exposed to direct sunligjt. One day of sunlight faded my own test subject to ots original color. Katlang stones sometimes take 2-3 to finish fading, though most are quicker.
Let us know what you find if you buy it. Seems like a nice stone but of course I just have a picture to go by.

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 Post subject: Re: Question about irradiated brown topaz
PostPosted: Thu Dec 20, 2018 9:06 pm 
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I have a last question regarding these stones which are photosensitive.
According to what I read in some places, the fading test can be realized with an electric light too, like an incandescent bulb ; does that mean that these topaz are sensitive to visible light in a general way, not to UV only ?
That would mean that a sensitive topaz might fade even under indoor lightning, right ?


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 Post subject: Re: Question about irradiated brown topaz
PostPosted: Fri Dec 28, 2018 2:08 am 
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Wow Stephen.....fantastic post! Lots of substance and full of great information to read. =D>

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 Post subject: Re: Question about irradiated brown topaz
PostPosted: Fri Jan 11, 2019 6:20 am 
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OK so I couldn't resist and bought it and here are 2 pictures of my sweet plump topaz under daylight and halogen.

I swear I didn't photoshop or anything, but for some reason my phone seems to want to capture only the reds and pinks, in reality the stone is more golden brownish orangy, especially in daylight.

So far it doesn't seem to have changed color since last time I saw it (that was 3 weeks ago) but probably it remained in a safe all that time, let's see what happens now. :-&


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 Post subject: Re: Question about irradiated brown topaz
PostPosted: Fri Jan 11, 2019 7:05 am 
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This is the most realistic picture I could get under halogen.


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 Post subject: Re: Question about irradiated brown topaz
PostPosted: Fri Jan 11, 2019 12:36 pm 
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Mmmmm, looks like a really nice stone. What's the pleochroism like? I'd guess a very strong pink one way.

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 Post subject: Re: Question about irradiated brown topaz
PostPosted: Fri Jan 11, 2019 5:22 pm 
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Brownish pink, strong orange, and a greenish yellow in the pleochroism under LED.
Under incandescent light the pink is stronger and the yellow more pure.


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 Post subject: Re: Question about irradiated brown topaz
PostPosted: Sat Jan 12, 2019 3:23 pm 
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Awesome!!!!


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