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 Post subject: Why Do Some Colored Stones Fade?
PostPosted: Mon Oct 11, 2021 6:07 pm 
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Hi! I'm hoping somebody can answer this question for me. I understand that stones that are colored by defects in the crystal structure can fade due to UV light repairing their structures, but why do crystals that are colored by impurities fade? Obviously the light is not removing the element(s) that causes the impurity. I'm sure it has something to do with the loss of electrons or something, but I just don't know, so if somebody could enlighten me, that would be great!


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 Post subject: Re: Why Do Some Colored Stones With Impurities Fade?
PostPosted: Mon Oct 11, 2021 9:20 pm 
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I think you must be talking about stones that are colored by trace elements, not impurities. Impurities are generally discussed when speaking of clarity.

Which stone colored by trace elements are you referring to?


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 Post subject: Re: Why Do Some Colored Stones With Impurities Fade?
PostPosted: Mon Oct 11, 2021 9:26 pm 
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1bwana1 wrote:
I think you must be talking about stones that are colored by trace elements, not impurities. Impurities are generally discussed when speaking of clarity.

Which stone colored by trace elements are you referring to?


I suppose I am referring to stones colored by trace elements. Mainly, I was thinking of stones like amethyst and rose quartz, but I've heard there are quite a lot of stones that fade when exposed to the sun. For example, amethyst is colored by iron, so why does it lighten when exposed to prolonged exposure to UV radiation?


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 Post subject: Re: Why Do Some Colored Stones With Impurities Fade?
PostPosted: Mon Oct 11, 2021 9:49 pm 
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Iron is necessary but not sufficient, it also needs the right structural defect which the iron apparently allows. Similar deal for phosphorous in pink (not rose) quartz, where the impurities allow a pink to red color to be acquired by exposure to radiation but aren't alone sufficient and the color can fade.
Rose quartz fading is variable per reports and a little unclear in general. Its color comes from inclusions of dumortierite (or something very similar), maybe I speculate in turn colored by manganese(??) In which case the manganese is probably popping down to +2 from +3 oxidation state as in kunzite(?????)

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 Post subject: Re: Why Do Some Colored Stones With Impurities Fade?
PostPosted: Tue Oct 12, 2021 1:12 am 
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SoulsOfWolves wrote:
1bwana1 wrote:
I think you must be talking about stones that are colored by trace elements, not impurities. Impurities are generally discussed when speaking of clarity.

Which stone colored by trace elements are you referring to?


I suppose I am referring to stones colored by trace elements. Mainly, I was thinking of stones like amethyst and rose quartz, but I've heard there are quite a lot of stones that fade when exposed to the sun. For example, amethyst is colored by iron, so why does it lighten when exposed to prolonged exposure to UV radiation?


Yes, amethyst has iron as a trace element. However the color of amethyst is caused by a natural radiation created color center in the iron (Fe 3+). Heat will destabilize (also UV light) and change the valance state of the Iron electrons. As the fe3 + becomes unstable the color center degrades and the stone fades. It is possible with heat to fade amethyst to any shade between where it started and colorless.

So Amethyst is not a pure trace element story. There are a lot of good articles on this on line.


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