Sapphires with unstable colors

[Stephen Challener]

There are a number of ways color can be generated in minerals. It's very difficult to nail down exact causes without a lot of research, and even then it doesn't seem to be a sure thing. Kurt Nissau did a heroic amount of work on this, and even after all that I think the picture he painted was a bit over-simplified in places (at least on brown topaz, which I think is more of a mess than he makes it out to be).
Both exposure to UV and exposure to heat can produce unstable colors in stones too.
In broad, imprecise terms, UV is energetic enough that you might expect it to be able to be absorbed and bump things up to an excited, metastable state, whereas lower energy radiation might just have enough energy to knock them back down but not to knock any back up.
Heat tends to push things into lower-density configurations while also shuffling things around. Shuffling things around randomly tends to make them fall into and stay in more energetically favorable configurations (like how vibration will level a sandcastle to flat sand)--but since it's also at a higher temperature, that favorable configuration might be different from what it is at room temperature, so while ruining some color-causing structures it might create others which are metastable on cooling.
But that's so vague as to almost be meaningless, though some of those keywords sound awfully nice.
Corundum is worse, since exposure to yet-higher energy radiation (x-rays/gamma rays) seems to fairly universally produce an unstable yellow color which fades with light exposure. So who even knows how that relates to some doing something similar with lower-energy radiation?? Not me!

[bluemlein]

Stephen - thanks for that. Given the current state of physics I wouldn't be surprised if someone said that the same sapphire in two different colours, can exist simultaneously in two places - thank you, quantum physics.
I've just found a fascinating site you all probably know already, https://www.gemstonemagnetism.com. Should be easy now that those super-strong magnets are available. I wonder whether there's a correlation between magnetism and colour stability.

[Barbra Voltaire, FGG]

Very well done site.
Kirk Feral is a member of our forum.

[Christopher P. Smith]

Hello all,

My apologies for the delay in uploading this. We have finally put to rest the whole topic of whether these unstable color centers remain after heating. It was generally felt that these defects were stabilized during the heat treatment process at relatively higher temperatures. Some even questioned whether this phenomenon was even possible after relatively lower temperature heating...

So for those of you who haven't seen this yet, that point is finally resolved. These unstable color centers in corundum can remain even after relatively higher temperature heating...

Best regards,
Christopher P. Smith

Heated sapphires with unstable colors_emailPDF.pdf

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