FMF - Friends of Minerals Forum, discussion and message board

[Elise]

[Jason]

I have a tanzanite..may have posted about it before....4 color pleochroism..maybe the only "quadchroic" stone there is

[Paul S]

Those are all very nice examples of pleochroism, well done and thank you everyone!

I am fascinated by the tanzanite from amethystguy, I have never read or heard about 4 colours, can you explain how this would be possible? I thought there was the maximum amount of colours, due to the fact there are only three axes in three dimensional space.

I was a bit confused after reading some more about this subject, but then I found out that we are all actually talking about two kinds of pleochroism:
1) by looking through a crystal along the different axes, different colours are visible.
2) by shining polarized light through a crystal and then changing its angle will change its colour.

These two are of course linked by the fact that pleochroic minerals are anisotropic, but this can be tested in the two aforementioned different ways. I am not sure though if both effects are always present in the same mineral or that one can be present without the other.

[Jason]

It was sent as a test sample stone to a gem lab and well known gemologist..he saw it online and contcted me wanting to test it. With the sudden influx of so called "mardi gras" tanzanite he wanted to determine if all the cut stones coming out of thailand were treated in some new or unknown way. After recieving the stone and running all manner of test on it as a base sample in the rough to then later on determine what the cut stones would show he said something to the effect in his 30+ years being a gemologist and testing, lookig, seeing all kinds of stones this was the first one he saw that showed 4 pleochroic colors

[Matt_Zukowski]

I am confused. What exactly does this stone do when you change its orientation? Does the blue area in the picture change to purple and the green area turn to yellow? Is the whole stone one color in some orientations? Does the green area in your picture go through one set of colors as you change orientation, and the blue area another?

One possibility is that you have a trichroic tanzanite with an area colored by something else, so that what you see are three colors depending upon orientation superimposed, in one area, on some other color due to something else?

Another possibility is that you have an xtal with different varieties of zoisite in different areas. I have a spodumene from Urucum that appears to be a single xtal but has different pink and green areas ( https://www.mineral-forum.com/message-board/viewtopic.php?p=5716#5716 ). I would think that it is more accurate to say the two areas display different pleochroism, not that i have a quad- or hexa-chroic xtal.

[Jason]

"I would think that it is more accurate to say the two areas display different pleochroism, not that i have a quad- or hexa-chroic xtal. "

Matt, when you can see all 4 colors with the dichroscope I think it's accurate to call it a quadchroic(don't know if that word exist since these crystals may be the only 4 pleochroic stone there is) ..heated to produce blue which is dichroic(since it's heated) and the green which is a different dichroic set..that equals 4 pleochroic colors through the dichoscope..

[John S. White]

Amethystguy - it appears that you did not answer Matt's question. Are some of the colors "fixed" or do all of them change with orientation? I suspect that the blue green shown in the photo may be due to something other than pleochroism, as suggested by Matt.

[Jason]

not really sure what you mean John? The crystal is blue and green..down the opposite axis it's purple and yellow..with a dichroscope it shows the face colors which you guys see in the picture but it also shows the dichroic properties of the blue which is purple down the c-axis and the green which is yellow down it's c-axis.....thats what a dichroscope does.
The crystal is colored by vanadium....it has been heated so it lost it's trichroic properties and now is like a typical tanzanite is..dichroic..if you take any regular tanzanite gemstone which is blue..when turned in the light it shows purple down the other axis..with a dichroscope you don't have to turn the stone..it shows both colors at the same time..so the blue portion shows purple down it's other axis..the green portion shows yellow down it's axis..
With both colors being in the same stone it shows 4 colors at the same time with a dichroscope..
even without a dichroscope showing 4 colors when orientated/turned by eye is a rare thing....can't think of any other that does..Robert James who used it and tested it said he has never seen another stone that did that except "mardi gras" tanzanite//zoisite which this was the first one he looked at. like this.....

[John S. White]

Okay let me try to make my point better. That the stone has been heated complicates the issue, but in my view pleochroism is revealed through a change in color when the stone is viewed from different directions, without the introduction of a dichroscope. It is evident that your crystal has distinct blue and green regions when viewed at right angles to the largest surface. When turned 90° about the long axis, do these colors change or do they remain the same? If the green remains the same then I suspect that we are not witnessing pleochroism.

Additionally, but unrelated to the above, are you quite certain that a treated tanzanite cannot show three colors? I have seen many crystals that show rich purple, magenta and blue, but I don't know if they have been heated or not.

[Duncan Miller]

[John S. White]

I agree, but I am still hoping for an answer to Matt's and now my question.

[Jason]

I still don't really get what your question is that i haven't already explained..
It's tanzanite..it is trichroic, it only becomes dichroic after heat treatment. heating zoisite can reduce pleochroism as in the case of tanzanite where the yellow, brown or green seen along the c-axis can turn blue upon treatment. which is what it has done on the blue side..the green side is has a different coloring agent..probably chromium..when viewed face up as in the pic it is blue on one half and green on the other half..when the stone is turned to look down the axis on the blue side and looking only through the blue side, not the long end into the green it turns purple. On the green side when viewed down the same axis as you looked at on the blue side the green area turns yellow(pale)...not down the long end but the short end.. The green does not remain the same it turns yellow when you turn the stone 90 degrees..it's tanzanite..it has been heated..regardless of the color of tanzanite pink, blue, yellow, green..it wll show trichoric properties..since it has been heated it has pretty much lost it's trichoric properties and is now dichroic on both the color..imagine if you cut the stone along the demarcation line between the blue and the green..wghat do you have..a blue tanzanite..it's other readily seen dichroic color is purple..now look at the green side..now you have a green tanzanite..when you turn tyhat stone it shows yellow as it's other readily seen dichroic color..now you just place them back together ..what does that give you??? 4 colors on one stone..just imagine a green and a blue tanzanite seperate then taking them and gluing them together..still will have there own dichroic colors..not when viewed down the axis that looks from the green into the blue but when the colors are side by side. With a dichoscope since it has a 90degree line if you run that line on the blue and on the green you can see 4 colors at once.
PINK : Pink, Green, Orange

GREEN : Green, Violet, yellow

YELLOW : Yellow, Violet, Greenish-yellow

ORANGE : Orange, Green, Violet

[John S. White]

Okay, I feel my question has been answered. All I wanted to know was what happened to the green area when turned 90° about the long axis of the crystal and now I know.

[Elise]

[Elise]

I am finally getting to unpack from the Springfield show. I was very happy to find a few slices of chiastolite which have "open" cores. Just rocking them back and forth over a non-polarized light source dramatically exhibits the eye-visible pleochroism which andalusite is famous for. Some of the inclusions radiating out from the core show the same color change and add color to the rim which is almost colorless otherwise.
Cheers!
Elise