New Mineral Named After GIA’s John Koivula
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 Post subject: Color Change Zircon - Does it exist?
PostPosted: Sat May 12, 2007 12:33 pm 
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Just wanted to field some opinions from the experts here. I had a rough dealer send me what they claimed to be a color change zircon of about 6cts rounded alluvial nodule. It changed from red to colorless. I thought it kinda odd, and always love unique gems, so I had then send it to me. After testing it (in my belief) it is a color change garnet. There was a very small find of these reddish to near colorless changing garnets in Africa I heard about. And I am sure this is what it is.

I did some research and found from the GIA a reference to:
"George Bosshart, a gemological researcher from Horgen, Switzerland, described a rare reverse color change zircon, which showed a dramatic 180 degree difference in color hue between violetish blue (in daylight) and bluish green (in incandescent light)."

Sounds nice!

Link: http://www.gia.edu/loupeonline/29962/27534/2674/current_issue_detail.cfm

Beforehand, I have never seen or heard of such a color change in this gem until that reference from GIA and this piece I got recently.

Wanted to see if anyone has seen any examples of this material, or can suggest a good way to test for color change in zircon?


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PostPosted: Sun May 13, 2007 6:24 am 
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The description in the article makes me wonder what they were seeing:

It reads:

"...reverse color change zircon, which showed a dramatic 180 degree difference in color hue between violetish blue (in daylight) and bluish green (in incandescent light)."

'Change' from violetish to greenish blue is not such a big change. Reminds me of the discussion about color change VS. shift on Richard Wises blog and recent thread here.

Not sure what the '180 degrees' in the phrase there try to say: just a metaphor? On a color wheel, that would a change between complementary colors - blue and orange - not between shades of blue.

On the other hand, violetish blue sounds unusual for zircon. They might be describing one of the very dark blue ones w/o much greenish tint. Haven't seen too many, and none that would hold its color very well in artificial light; getting some of the expected graish-gren mask in lower light.


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PostPosted: Sun May 13, 2007 2:09 pm 
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Not sure what they meant by 180 degrees either. I found the full reference to this stone that was presented at some GIA conference. The explanation is:

Quote:
"This phenomenon is opposite to the color
change of alexandrite, which displays green hues in daylight and purple hues in incandescent light. CIELAB color analysis of this zircon with a Zeiss MCS 311 multichannel color spectrometer revealed a 75° change in hue angle when recorded in a direction parallel to the optic axis (approximately parallel to the table facet). In a direction perpendicular to the optic axis, the change in hue angle was 65°."


See page 11: http://lgdl.gia.edu/pdfs/grcprogram.pdf

They theorize about what makes it color change here:

"The reverse alexandrite effect of this zircon is due to uncommon and strongly polarized absorption features in the visible region of the spectrum. They consisted of at least 10 multiband absorption maxima dispersed across the entire 400–700 nm range (and of another eight groups of bands up to 1800 nm in the near-infrared region). The absorption peaks located at 656/661, 590, and 683/691 nm"


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PostPosted: Sun May 13, 2007 5:07 pm 
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I think that the 180 degrees out is a reference to the green in incandescent and violet in natural light which is the opposite of alexandrite.


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PostPosted: Mon May 14, 2007 5:50 am 
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hi ben. in 40 years i've never seen a color change zircon. since thy apparently exist i wish you success in your hunt for one.

i'm a zircon nut myself. ever since i heat treated two 20 /30 carat em. cuts years ago. put them right in the fire.

an old friend had an amazing colletion. one huge lot. really big. all large stones. all kinds of exotic colors. but they were decaying. so they were all cloudy. heartbreaker that was.

I have a 4 carat round green. don't know if i mentioned it to you when we were in ny. think i did.

i can ask around but if someone has one they might not be selling it!


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PostPosted: Mon May 14, 2007 2:43 pm 
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Whether their true color changers I don't know, but www.cresla.com has had & may still have Zircons they claim change color.

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PostPosted: Mon May 14, 2007 5:56 pm 
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Hi all,

Interesting discussion, the red to colorless change sounds fascinating.

I had never encountered any change of color in Zircon until recently. I picked up an unusual orange Zircon (think slightly reddish orange Spessartite) that actually darkens in sunlight. Naturally I couldn't wait to give it a shot and it actually does get considerably darker in tone and takes on a brownish a modifier after exposure to direct sunlight. Very distinct.

I suppose this is a kind of tenebresence as opposed to proper color change as it takes several hours or even days for the the original orange to return. The stone is from Cambodia and my source on this one mentioned that he has seen these a couple of times in several decades in the business.

Unfortunately I haven't really got any decent photos of this to share, I should try to shoot it again.

Brad

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PostPosted: Mon May 14, 2007 6:23 pm 
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Hi Brad,

Would love to see some pictures of the stones. Your stuff usually comes out quite well. I have only seen some poor pictures of material like you describe. I don't know if they were the real deal or not. However, the stone mentioned in the GIA article sounds enchanting. Purplish blue to bluish green...

The cc garnet changing to colorless is another oddity. I had heard about them from another rough dealer last year who wasn't selling his. Luckily this mistake ended up in my favor, as I was curious about this stone.

I will admit its not 100% colorless, its has the very slightest almost unnoticed greyish modifer. I have not got it cut yet, so it could turn out something completely different.


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PostPosted: Mon May 14, 2007 6:27 pm 
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Forgot to mention, here is a list of locations of color change zircon showing up from research and people mentioning stones from specific areas.

Cambodia
Sri Lanka
Mogok

Where is Africa!?


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PostPosted: Tue May 15, 2007 8:00 am 
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Quote:
I had never encountered any change of color in Zircon until recently. I picked up an unusual orange Zircon (think slightly reddish orange Spessartite) that actually darkens in sunlight. Naturally I couldn't wait to give it a shot and it actually does get considerably darker in tone and takes on a brownish a modifier after exposure to direct sunlight. Very distinct.

I suppose this is a kind of tenebresence as opposed to proper color change as it takes several hours or even days for the the original orange to return.


Very interesting -- I just happened to pick up a little parcel of zircon from a local hobby cutter who is not cutting any more (unfortunately, his eyes are going). Included is a 6.6ct rough (labeled as from Tanzania), that is pretty much just as you describe, but mine goes from a wonderful reddish orange to almost completely colorless in sunlight. I've exposed it to sunlight twice now -- the first time took about 2 weeks for the orange to return. I did it again this weekend to show the previous owner the effect.

The first time I exposed it to sunlight, quite by accident, the change was a little hard to discern because the rough has some iron oxide staining that give the whole thing that rust orange color. My first impression was that the stone had darkened -- but close inspection shows the stone itself to be colorless and I was only seeing the iron oxide. Here's a pic of the faded rough:

Image

All of the orange you see here is concentrated in small fissures on the surface of the stone. It will certainly cut out.

I hadn't planned on posting this, so I don't have a "before" shot. I'll try to do that when the color comes back. (Hopefully it comes back.... ;-) )

If it helps any of you long-time rough guys, the paper is marked as being purchased from "Lochs" in June of 1989. I don't know if that supports the "Tanzania" label or not.

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PostPosted: Tue May 15, 2007 5:40 pm 
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I had these shots on hand....as I mentioned I need to spend a bit more time capturing the effect. The first image is pre-exposure to sunlight.

Image Image

Brad

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PostPosted: Tue May 15, 2007 8:47 pm 
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Wow, that looks amazing! Like the cut. Thanks for posting!


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 Post subject: Re: Color Change Zircon - Does it exist?
PostPosted: Thu May 09, 2019 10:16 am 
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We had recieved a reverse color change zircon to our lab and currently we are conducting an elemental analysis. This zircon is from Madagascar, weight: 10ct.

Violet in daylight change to blue-green in incandescent light.

Should you have any past article relate to this topic, appreciate if you could suggest me the links.

Will keep you all informed.


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 Post subject: Re: Color Change Zircon - Does it exist?
PostPosted: Tue May 21, 2019 12:18 am 
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I have written extensively on my website brucefrytourmaline.com and posted on GO about a reverse color change cuprian tourmaline. I call it Laurellite and it has the same color shift that the zircon has. It is a strong color changer at 90 degrees. I have had the gems tested by three independent labs and nothing unusual was found with their chemical make up. Their absorption spectra do NOT have multiple narrow peaks or sharp features. You can have them change color by viewing them in natural light and then with natural light that went threw a simple sheet of primary yellow gelatin. I have tried for over 10 years to get more research done on them with little success. I could send you a complete absorption graph from my spectrometer if you want to compare your gem's color change. Good luck with your project.

Bruce


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