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 Post subject: Color change stones - single or double refractive
PostPosted: Sun Feb 19, 2012 7:56 pm 
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I love color change stones, especially garnets - the new blue Massasi color change- but I love them all. I am confused because garnets are single refractive. Spinel is also single refractive, and also has a color change variety. Utilizing a calcite dichroscope to identify refraction, single refractive stones show only one color. Shouldn't color change stones show two colors therefore be double refractive?

Looking for clarity in California,
Peach


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 Post subject: Re: Color change stones - single or double refractive
PostPosted: Sun Feb 19, 2012 8:43 pm 
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I'm someone can expand on this further, but a color change stone is not replying on optical properties for its color change. Being SR or DR is an optical property dependent on the crystal's internal structure, where color change depends on chemistry of the material. Certain coloring agents react differently to the different light sources at different areas in the color spectrum.

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 Post subject: Re: Color change stones - single or double refractive
PostPosted: Sun Feb 19, 2012 10:19 pm 
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From the Gemology Project
Quote:
The basis of the change in color lies in the color of the light that is used. Part of the light is absorbed and part is reflected. The color is depending on our perception, the absorption and the type of light used. Daylight has more blue in its spectrum than incandescent light. Alexandrite has a big absorption in the green part of the spectrum and reflection in the blue and the red part. If viewed in daylight the red portion of the spectrum is absorbed by the stone and the blue and green part is reflected to the observer.Therefor the stone appears green. If incandescent light is used this is more yellow. We as humans correct the color of things by our mind so the paper that looked white in sunlight still looks white in incandescent light. But in reality is a bit yellowish. Never the less there is less blue in the spectrum. So the reflected light also has less blue in it. So the reflection to the observer is less blue and the same amount of red. The result is that there is not enough blue reflected to mix the perceived color to green. So the perceived color is more reddish. The less blue is in the light the more red the stone appears.


The above could use extensive improvement, but the overall concept is correct.
Anyone want to update the page?


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 Post subject: Re: Color change stones - single or double refractive
PostPosted: Sun Feb 19, 2012 10:43 pm 
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I understand the perception of the stone color to our eyes in different light. Is this then a chemical in the structure of the color change stone as opposed to non-color change stones that causes this reaction to different light sources that we perceive?

Peach


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 Post subject: Re: Color change stones - single or double refractive
PostPosted: Sun Feb 19, 2012 11:01 pm 
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It seems certain elements, chromium and vanadium, in trace amounts, cause significant absorption of certain portions of the spectrum, transmitting the rest.


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 Post subject: Re: Color change stones - single or double refractive
PostPosted: Tue Feb 21, 2012 8:58 am 
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I will try to present the basics of color change in gemstones, but first a little background.

I am a cutter and lover of all colors in tourmaline. In search of the illusive colors of tourmaline, to complete my color wheel, I purchased rough from Mozambique that turned out to be both cuprian and a strong color changer. In search of understanding concerning the chemistry and color change ability of the new variety of tourmaline I have come to call Laurellite, I had both the GIA and an acknowledged expert work on the gemstones that I cut from the rough. I have noted the papers in earlier threads that you might wish to look up. The reverse Alexandrite nature of the color change in Laurellite has put the traditional model of color change in gemstones/all materials in question.

The following basic definition of color change is taken out of the conclusion to the article I helped write for the British Gemmological Laboratory article.

"The alexandrite effect (color change) is a non-colour-constancy phenomenon caused by a combination of chromatic adaptations to the different light sources and vision system responses to the spectral distribution of the light emitted by the alexandrite effect gemstones illuminated by the corresponding light sources.

Color change put simply, DIFFERENT COLORS CAUSED BY DIFFERENT WHITE LIGHT SOURCES. This phenomenon is not caused by different areas of color in a gemstone as seen by different directions of view or zones of color in the gemstone, when view under a single light source. The change in color must be a result of different (WHITE) lights, which requires that the absorption spectrum of the gemstone must have at least two distinct peaks of absorption. This type of absorption curve can come from one trace element, a color center, or any combination of chromophores. Now, the most critical step in color change is what the eye/mind does with the color information sensed by the eye.

The eye/mind evolved to obtain information from the environment that is useful to man. One of the eye/minds efforts is to maintain color consistency. Fruit such as oranges should still look orange under the changing spectral mixtures (color) of light found in a day's light (10 AM is different then 3 PM etc.). The eye/mind is very good at color consistency, but not perfect. It is the break down in the eye/mind's ability to maintain color consistency, that leads to color change, not just the accurate sensing of differences in the balance of wavelengths, by the absorption of light by the gemstone, from different (white) light sources.

For a second helping richer in details, send me a PM and I can supply a copy of the article in the British Gemmological Journal.

Bruce


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 Post subject: Re: Color change stones - single or double refractive
PostPosted: Tue Feb 21, 2012 11:31 am 
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Gems4Peach can't use the PM function yet.
Specifically what issue are you referring to?
I think we'd all like to know. :D


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 Post subject: Re: Color change stones - single or double refractive
PostPosted: Tue Feb 21, 2012 11:58 am 
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bruce_tourm wrote:
The eye/mind evolved ...


Now THAT is something I'd like ANYONE to try to explain 8) .


P.S. -- Even Darwin couldn't do it.

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 Post subject: Re: Color change stones - single or double refractive
PostPosted: Tue Feb 21, 2012 3:40 pm 
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Here goes:

1, One of the biggest problems we had in publishing the color research on Laurellite was how much of a change you had to have in the hue angle in the CIELAB color world to be a color changer. It was finally accepted that a 20 per cent change was enough between the illuminant that approximates sunlight and the illuminant that approximates incandescent light.

2, You can have more than one peak that participates in the color change effect. Some color change garnet shows this effect.

3, You can have no change between the illuminants that I mentioned in 1, and still have a color change between other illuminants in the CIELAB color world.

4, How a gemstone is cut and its orientation can effect color change. Alexandrite definitely has a perfered axis for showing color change. Again this color change is dependant on different "white" lightrs and not just a different axis of view.

5. All purple gemstones shift quite a bit with different sources of white light, but are not called color changers because of the universal nature of the change. To be called a color changer, the effect must be an exception to the color effect seen in "normal" examples of the gemstone. That is unless color change is in the definition of the variety such as alexandrite.

6. One of the reasons that the best alexandrite shows such an exceptional color change is that the color formed by mixing the green/blue with the red in mixed light is yellow which the eye tends to loose in the two principal colors. This is in stark contrast with, lets say blue to green, which would be dominated by blue/green in most normal situations that are dominated by mixed light from natural and yellowish indoor lighting. Therefor you do not see much difference between a true blue green tourmaline and a blue to green color changer unless you are sensitive to the lighting being used.

Since I brought up blue to green as a color changer in tourmaline, I should probably tell you a story. It is a real life adventure story in the colorful world of tourmaline. I have a very dedicated goldsmith as a friend and he likes my collection of tourmaline, so I share my new ones with him. Right in the middle of the production of cuprian tourmaline from Mozambique I cut a rather incuded round that did not really fire my imagination in color. I really did not know if it had copper as a chromophore in it or not, but it was still interesting to one who seaks different shades of color in tourmaline. Because the tourmaline was included, I had to look at it carefully and to work with it more (missed the color change completely) to get a decent stone and as always I waited to see a finished gemstone before judging its color. It looked to be a pretty decent blue under my work lights, but it change to a definite green outside on one of Pennsylvania's gray days. Now sometimes I really don't trust my eyes to see the truth when compared with what I want to see. So I gave it to my friend without any warning. He admired the blue color and was nice enough to get up and go outside to look for any change in the tourmaline's color. I did not go out and try to influence him, but sat back and heard a roar that the tourmaline was now completely green. I now know that the round is cuprian from using my spectrometer and I have found the cuprian tourmaline in general tends to be more unstable in color (particularly unheated ones) than other varieties of tourmaline.

7. Gray and brown are not spectral colors and if a red for example is desaturated to brown by a change in lighting, that is not a color change effect. Combination colors such as gray are represented at the origin of the CIELAB color graph and do not effect the hue angle.

8. A most unusual tourmaline displays the color effect called Usambala. The effect is caused by the non-linear absorption of the spectral colors as they pass threw the chrome colored Dravite tourmaline. The effect is quite dramatic, a rich red to a rich green, but it depends on a single light source and different thicknesses of material and therefore is not a color change gemstone under this discussions definition.

9. Many color change gemstones need a structured set up to really appreciate the effect. Good lighting in either pure natural light that is not too yellow or low temperature incandescent. None of the rough Laurellite that I bought was sold as a color changer and I did not know the first piece was a color change until I went to take it for a walk. I work under incandescent in the winter and the first rough piece was a heavely ground, rather crude, crystal of tourmaline. As I ground off the remaining reddish skin, that I think comes from natural radiation effects on manganese, the crystal stayed blue gray. I was disappointed because I was looking for a purplish blue, the rarest color in the tourmaline world. I will never forget looking down at the oval, after cleaning it up, as I went out the front door and saw that it had turned violet. The second burst of discovery came over a year later when I received the piece of rough that would become my best example of Laurellite. It has much less gray in it and goes from a strong violet (natural) to a blue with a touch of green (incandescent).

10. The reverse alexandrite nature of Laurellite's color change is a "smoking gun" when it comes to proving that the tradition explination of color in gemstones can NOT be correct. But the example of ruby, alexandrite and emerald are amenable to such a simple explination of differences in the spectral distribution of the light sources (natural light less red and incandescent more red) that I have found the level of understanding of color change by "experts" to been completely uneffected by the discovery of Laurellite.

11. This is kind of a foot note, but purple blue to purple cuprian tourmaline can be heated to a much more expensive color for tourmaline. The color is cyan and is know in the trade as paraiba type. I have no idea how many Laurellites have been heat to produce this highly valued gemstone, but I am sure that Laurellite is being heated to oblivion right now since very little material is coming out of mozambique any more and there is no commercial reason to save this unique example of a reverse alexandrite color changer. Part of the reason I contribute to the Forum is to try and raise the awareness level of interested people about color and color change in tourmaline in general and Laurellite in particular.

Bruce


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 Post subject: Re: Color change stones - single or double refractive
PostPosted: Wed Feb 22, 2012 12:42 am 
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Bruce,

Thank you so much for the information, I have found it intriguing. So much so that I want to learn more. I will be doing serious research on the subject. There is so much more to color change than meets the eye!

Thanks,

Peach


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 Post subject: Re: Color change stones - single or double refractive
PostPosted: Wed Feb 22, 2012 7:06 am 
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Peach,

I hope you share what you find. I have not paid for papers on the inter net, but I have a research contact that can supply them since I am working with him on more research on Laurellite's composition.

Good Luck

Bruce


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 Post subject: Re: Color change stones - single or double refractive
PostPosted: Wed Feb 22, 2012 11:27 am 
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There are several articles in G&G on the mechanism of Color Change
Cause of Color Change
(Gübelin)W82:197-206,
(Fritsch)Su88:81-102

and several gems which display color change with articles describing them in G&G:

in bastnäsite from Pakistan (GNI)Su07:165-166
clinochlore from Russia (GN)Su95:129-130
corundum—(Gübelin)W82:197- 206; with asterism (GN)W87:241
diaspore (LN)Sp87:44-45 fluorite (Gübelin)W82:197-206,
fluorite (Gübelin)W82:197-206,(GNI)Su06:173-174, F08:263;
from Ethiopia (GNI)Su07:168-169
garnet—(Stockton)Su82:100-101,W85:205-218, (Gübelin)W82: 197-206, (Let)Su85:116, (GN)Su98:138, F98:222-223;
grossular (Manson)W82:204-213;
grossular-andradite, from Mali (LN)Su03:145-146; from Kenya (GNI)F09:223-224;
from Madagascar (Schmetzer) W99:196-201, (GNI)Su03:156;
pyrope-spessartine (Manson) W84:200-207, (GN)Sp96: 53, W96:285-286;
from Tanzania (Dirlam)Su92:80-102
garnet inclusion in diamond (LN)F82:169
in glass imitation of alexandrite (GNI)Sp04:73-74
in hackmanite, produced by UV radiation (GN)Su89:112, W89:245-246, Su92:134
influenced by type of illumination (Fritsch)F87:126-139
kyanite (Gübelin)W82:197-206 monazite (Gübelin)W82:197-206 photochromic, of artificial glass
(GNI)Sp09:72-74
in Purple Zandrite glass(GNI)W05:364-365
remondite-(Ce) from Canada
(GN)W92:270-271
reverse, in zircon from Myanmar(Bosshart)F06:94 sapphire—(P. Keller)Sp85:20-25;
with lead-glass filling (GNI)Sp08:88; from Queensland, Australia (GN)W93:292; from Sri Lanka (GN)Su88:121; star (GN)W87:241; from Tanzania (Dirlam)Su92:80-102, (GN)Sp95:64-65
in sapphire/synthetic color-change sapphire doublets (LN)Su03:149- 150
in synthetic alexandrite from Russia (Schmetzer)F96:186-202
in synthetic apatite (LN)Sp01:57 in synthetic diamond
(LN)W98:286-287
in synthetic sapphire (LN)Su95:127,
(McClure)F10:218-240
spinel—(Gübelin)W82:197-206, (LN)Sp83:48, W84:232-233;
cobalt-colored (Shigley) Sp84:34-41, (LN)F90:226-227, (GN)W90:305-306;
and synthetic spinel (LN)W85:236-237, Su91:112-113
tourmaline—(GN)F91:184-185; from Mozambique (LN)F04:250- 251, Su05:173-175
in triphylite from Brazil (GNI)F09:229-230 [erratum (GNI)W09:311]
in Zandrite glass (GNI)Sp04:73-74 [erratum W05:369]
zircon (GN)F95:212-213


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