[Rockhounds] Why does the color of some gems fade?

[Axel Emmermann]

This is a layman trying to explain quite complicated stuff. Please seek
confirmation with professionals.
Still, here's what I believe can happen:

Some elements (some rare earths for example) may change valence when exposed
to light.
The pink and green calcite cleavage  rhombs from Chihuahua fade to yellow
due to prolonged exposure to sunlight. In this case one divalent RE-ion
could give an electron (kicked off by a photon) to a trivalent RE-ion. The
first would then become divalent and the latter divalent. Many ions change
color due to valence. Fe2+ is green, Fe3+ is brown... Cr3+ is green, Cr+6 is
orange...

Green fluorite may fade to gray due to annealing of f-centers or e-centers.
Electrons that are kicked of their ions in a crystal lattice may cause
coloration. The energy of photons in the sunlight can be sufficiently high
to kick them back into place. I think that the same mechanism can change
lilac kunzite into green hiddenite... A short stay in a nuclear reactor
would return the color (I'm sure this is the only reason why Iran should
want a reactor ;-)))
Same like radioactive rocks can turn quartz into smoky quartz...

Hope his helps?

Axel


> -----Oorspronkelijk bericht-----
> Van: rockhounds-bounces at lists.drizzle.com 
> [mailto:rockhounds-bounces at lists.drizzle.com] Namens jerrybs at frii.com
> Verzonden: dinsdag 9 oktober 2007 4:33
> Aan: Rockhounds at drizzle.com: A mailing list for rock and gem 
> collectors
> Onderwerp: Re: [Rockhounds] Why does the color of some gems fade?
> 
> Some of the opal, opalite, etc will loose their blue color 
> when they dry out.  I've also seen this in thundereggs.
> 
> Jerry
> WA
> 
> 
> 
> > Carol Carter-Wientjes wrote:
> >>
> >> Hi Rock Friends,
> >>
> >> Just back from a trip to Oregon and a first sunstone dig. 
> What fun! 
> >> We didn't find anything to retire on but did recover a 
> couple lbs. of 
> >> small schillers and a few little reds. Easy to see why people get 
> >> hooked on digging them, count me in!
> >>
> >> On to the question.....Why do some pink and purple gems fade? ie, 
> >> rose quartz, Holley Blue, etc. How about carnelian too? 
> Does it make 
> >> a difference if they're heat-treated?
> >>
> >> Looking forward to your comments on this, CaroL
> >
> > Sunlight provides the energy needed to change crystal 
> structure at the 
> > molecular level and alter the properties of many colored crystals.
> >
> > Kreigh
> >
> > --
> 
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