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 Post subject: Sapphires with unstable colors
PostPosted: Wed Jun 12, 2019 9:17 am 
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Hello all,

I thought the GemologyOnline community may find this topic interesting. I should have added this to the string of topics when we released this, but better late than never...

My thoughts behind developing the disclosure as it ended up, was to try and make these stones more intriguing as opposed to alarming. This is a natural phenomenon and both colors tend to be rather attractive. So in an attempt to help support the industry with how to deal with such stones, we came up with the policy in the accompanying link:
Attachment:
ColorStabilityTesting2019_PressRelease.pdf [3.78 MiB]
Downloaded 8 times


During the course of this work and what we encounter on a daily basis, it is actually quite surprising how many stones have colors that can shift under these different conditions.

Best regards and I am always interested in feedback on the policies of AGL, so please feel free to comment and I will try my best to respond.
Christopher P. Smith


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 Post subject: Re: Sapphires with unstable colors
PostPosted: Wed Jun 12, 2019 9:30 am 
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This is interesting, though it seems a roundabout way of describing tenebrescence. How does this relate to the well-known phenomenon of normal unstable, permanently fading yellow tones induced by natural or artificial irradiation of sapphire?

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 Post subject: Re: Sapphires with unstable colors
PostPosted: Wed Jun 12, 2019 12:19 pm 
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Pdf of the Krzemnicki article sited in the Christopher Smith pdf above:


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 Post subject: Re: Sapphires with unstable colors
PostPosted: Wed Jun 12, 2019 1:50 pm 
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Thanks for the link. This subject is very interesting, but calling the color 'unstable', which as far as I know has always been used to refer to irreversible fading, instead of using the correct, already established term for this phenomenon (tenebrescent) is vexing. The fact that actual unstable, permanently fading yellow sapphire exists makes it even more of a headache.
That being said, I wonder if a portion of those permanently fading stones are actually tenebrescent instead. Though iirc almost any sapphire will go unstable yellow with enough gamma rays? It has been a while since I dug into that.

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 Post subject: Re: Sapphires with unstable colors
PostPosted: Wed Jun 12, 2019 1:54 pm 
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You know Stephen, I agree with you.


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 Post subject: Re: Sapphires with unstable colors
PostPosted: Wed Jun 12, 2019 7:52 pm 
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Me too.


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 Post subject: Re: Sapphires with unstable colors
PostPosted: Thu Jun 13, 2019 8:47 am 
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Hi Stephen,

I am really glad you made your remarks. I don't know why I didn't think of that.... Throughout my time in gemology, we always referred to this phenomenon in corundum as an 'unstable color'. But tenebrescence is the correct term. We apply this term correctly for other minerals. I wonder why we haven't for corundum??

I think I will reconsider our description and disclosure for this material.

Why didn't I post this before putting the policy in place! LOL See, GemologyOnline is a phenomenal resource for literally anyone interested in the field of gemology!

Thank you and best regards,
Christopher P. Smith


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 Post subject: Re: Sapphires with unstable colors
PostPosted: Thu Jun 13, 2019 8:59 am 
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Stephen,

A follow-up question. I have not heated a hackmanite, or sodalite, etc which possesses tenebrescence. References indicate the effect is destroyed by heating.

With corundum, the color just reverts back to its inactive or stable color with low level heating/warming. However the effect of activating the color center can be redone virtually endlessly.

The activated color-center only becomes stable once the stone is heated at relatively higher temperatures.

Tenebrescence in these other minerals is much more sensitive and reactive, but I don't know if you've ever experimented with this.

Best regards,
Christopher P. Smith


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 Post subject: Re: Sapphires with unstable colors
PostPosted: Thu Jun 13, 2019 12:57 pm 
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As someone who has been fascinated with tenebrescence for the last decade+, I have collected everything I can which is tenebrescent.
My favorite is the orange zircon which goes beige in light and reverts to tangerine when either heated OR put back in the dark.
viewtopic.php?t=9562

and
https://gemologyonline.com/hackmanite.html


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 Post subject: Re: Sapphires with unstable colors
PostPosted: Thu Jun 13, 2019 1:14 pm 
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Hi Barbra,

Thanks for the link. However this statement I guess answers my question to Stephen: "This photochromic effect can be repeated indefinitely, although any heating of the mineral destroys tenebrescence forever."

This would make me reconsider calling the phenomenon in corundum as tenebrescence, because the unstable color-center does survive the relatively low temperature heating that is sometimes employed to treat corundum. Although heating at relatively higher temperatures will stabilize the color-center in its 'active' state.

I guess this needs further consideration....

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Chris


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 Post subject: Re: Sapphires with unstable colors
PostPosted: Thu Jun 13, 2019 1:22 pm 
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Tenebrescence is weird, with different featires and reaponses to treatment. Tenebrescent scapolite, for instance, becomes way more tenebrescent after irradiation, for some reason. I almost wonder if something similar could hapoen with corundum too, given it does turn yellow with gamma rays.

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