Sapphires with unstable colors

[Christopher P. Smith]

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:

ColorStabilityTesting2019_PressRelease.pdf

(3.77 MiB) Downloaded 768 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

[Stephen Challener]

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?

[Barbra Voltaire, FGG]

Pdf of the Krzemnicki article sited in the Christopher Smith pdf above:

[Stephen Challener]

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.

[Barbra Voltaire, FGG]

You know Stephen, I agree with you.

[1bwana1]

Me too.

[Christopher P. Smith]

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

[Christopher P. Smith]

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

[Barbra Voltaire, FGG]

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.
https://www.gemologyonline.com/Forum/ph ... php?t=9562

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

[Christopher P. Smith]

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....

Best regards,
Chris

[Stephen Challener]

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.

[path914]

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.
https://www.gemologyonline.com/Forum/ph ... php?t=9562

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

I am also a huge fan of this reaction and have been collecting whatever good examples I can find over the years. I have several Burmese hackmanite and a rather large (17ct+) zircon which goes from a fantastic orange to less fantastic yellowish-brown within minutes

[Barbra Voltaire, FGG]

You know, Paul, I can kind of get my mind around a color change by adding energy....like light and heat, but what has always perplexed me is the color change obtained by putting the stones in the dark.....

[path914]

Me too Barbra. One of these days, I should go over and ask some of the labbies about it. After all, they sit only about 50 yards away

Every time I pull the zircon out of its parcel paper, I lament the fact that the amazing color disappears so quickly. Probably why I have never gotten around to setting the stone. On the other hand, my wife absolutely loves the hackmanite and is always on me to look for more in Tucson each year.

[bluemlein]

once the sapphires (example given) have got their suntan, so to speak, how long does it take for them to revert to their "normal" colour? are there any artificial tanning preparations? (LOL - of course there are) what fascinates me is that UV light has its own temperature and there are low-temp as well as "normal" temp UV lights.

Low-temp UV light is useful in disinfecting water in treatment plants etc and curing/drying printing inks. Most UV radiation is high-energy; IR are longer waves than UV. Infra-red causes a rise in temperature in the object subjected to it, tho I didn't see that IR has any effect on the colour. Then what is it, specifically, in the UV that causes such excitability? has it to do with the length of the waves?

Since UV sits next to Xrays on the spectrum, is there any thought as to what the inactive centre might do when it finds itself being probed by Xrays? (which are shorter than UV.) is the temp. of the Xrays too cool to have any effect?
or is it the actual physical pass-through of certain frequencies of light what causes the change?

If I have misunderstood some of what is going on here, please set me straight. In the past it was the optics that killed me.