Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
I was just thumbing through my copy of Gemstones, Symbols of Beauty and Power by Gubelin and came across something I hadn't heard before.
Page 114: Tanzanite, Blue Patrician from Black Africa:
Quote:
The aqueous calcium-aluminum silicate has strong pleochroism. Depending on direction, tanzanite is either violet-blue, or an unmistakable amethyst color or simply brown. The undesirable brown and violet shades are only removed after professional heating and the radiant cornflower blue is brought to full expression. If this delicate process is only partially successful, a violet tint is left behind that is particularly visible under artificial light. No other gemstone posses this unique ability to rid itself of two of its triple colors by means of heat treatment. If only one of the three colors, frequently the brown, is eliminated as a result of unprofessional heating, a color changing tanzanite is created, similar to color changing sapphire or spinel. In day light it looks blue, in artificial light it looks violet.
So, of course I had to take my tanzanites outside...yes blue indeed and then in the closet with a candle...and they were VIOLET with strong red flashes! I had never realized. This is super cool. I am so glad my stones were unprofessionally heated!
Is this something everyone is aware of and I've been living in a remote cave?
Im not sure that it has anything to do with "professional" or non professional heating. I have (had) a few un heated tanzanites that were naturally blue, and this color change is very pronounced in them. I also heated a yellowish brown piece, that was on the light side, and the same color change is true, although the finished color of this stone is light purple/blue.
I have on un cut, unheated piece that I'll photograph and post that clearly shows 3 distinct colors.
Joined: Sun Oct 16, 2005 4:25 pm Posts: 161 Location: Michigan, USA
I've noticed that the nicer pieces I've practiced with did this blue/violet color change but assumed it was a just a standard property of tanzanite. (I have to admit I have the book but have read only parts of it, as I have a hard time getting past the gorgeous photos!)
Joined: Mon Oct 17, 2005 10:56 am Posts: 6461 Location: The frozen north prairie :-/
I remember the first time I finally took my tanzanites outside in the sunshine! They went from what I thought was a pleasant violet color to blue! I try to be sure to take all my colored stones into the sunlight now ... you'd be surprised at what you find out .
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
AS I was standing in the closet with a candle , I thought, wouldn't it be handy if someone developed a small light box that would mimic daylight and incandescent lighting solely for the purpose of color change.
I suggest the lighting unit fit where the LW+SW UV light goes on the UV light box.
That may be good for portability issues if you can find an incandescent light source for it.
For the lab, I have the exact same unit as the one first pictured on that Kassoy page.
Notice how the LWSW bulbs face down into the cabinet. Now if the top of the light unit had a similar bulb arrangement for incandescent/fluorescent, independently switched, how easy would that be? No bulbs to change.
The enclosed cabinet would ensure no extraneous light would be involved.
Joined: Mon Oct 17, 2005 10:56 am Posts: 6461 Location: The frozen north prairie :-/
Great idea, JB, but until the inventors get going--IF one could find an incandescent bulb shaped like a flourescent bulb, could they be used in place of the LWSW UV lights in your cabinet? I realize that wouldn't exactly be portable, but for lab use, as you said.
This would probably have to be a do-it-yourself set-up, since the use of such an item would most likely be limited to a small buying group.
Joined: Wed Apr 26, 2006 12:44 am Posts: 2056 Location: San Francisco
The heat treatment sounds pretty simple.. put the cut stone in investment casting, w/ 3 " thick all around.. heat to 1200 degrees ramping 100-200 deg/ hour and same for cool down, and voila.. from the guys that sold me some excellent rough I haven't cut yet.. (tom schneider gems).
Joined: Fri Feb 24, 2006 1:20 am Posts: 2756 Location: Southern California, U.S.A.
jleb wrote:
The heat treatment sounds pretty simple.. put the cut stone in investment casting, w/ 3 " thick all around.. heat to 1200 degrees ramping 100-200 deg/ hour and same for cool down, and voila..
It is and I've done it several times. The color change seems to depend on the intrinsic nature of the material being treated. My observation is that the darker the original brown color, the darker the tone and saturation of the blue/purple color. When finished I've always just turned the oven off at 1200 and let it sit unopened for at least 12 hours instead of ramping down. No problems yet.
I have issues with this topic and haven't posted to the thread previously because my book research hasn't yet resolved some of my questions. Does anyone have any cites as to how heat actually changes blue zoisite's pleochroism? What happens on an atomic level?
Joined: Mon Oct 17, 2005 10:56 am Posts: 6461 Location: The frozen north prairie :-/
I don't know why my brain wasn't working earlier in this thread, but I just remembered -- someone does make a box with both flourescent and incandescent lights. I saw quite a few at the Tucson show ... they were used to show the color change in zandrites, garnets, and alexandrite. Don't know where they obtained the boxes, but they did look rather professional, if memory serves me correctly (which, as you all know, it doesn't always ).
Does anyone have any cites as to how heat actually changes blue zoisite's pleochroism? What happens on an atomic level?
That was a good question ROM. I've searched quite a bit and so far the only site that even hinted at the mechanism stated that heat alters the yellow green trichroic crystal axis leaving a more face up blue color.
As to what happens in this chemical alteration....don't know yet.
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