FMF - Friends of Minerals Forum, discussion and message board

[Roger Warin]

Hello, about tanzanite pleochroism

I enjoyed doing this little manipulation (“manip”), as they say in chemistry.
On my last visit to the Mineralientage (Munich), I didn’t only see Masterpieces.
I bought a tiny specimen of strictly colorless zoisite for €100, with a Tanzanite label.
It is becoming difficult to find an unheated specimen.
The specimen was purchased from a stand selling stones for cutting.
In gemology, these unheated crystals regain a high value compared to the “sapphire” tanzanites that have been so numerous since the end of the 1980s.
Is my crystal a zoisite or its tanzanite variety, since it would have turned blue after heating? At €100 per gram, we can hope so.
I examined the trichroism of this zoisite, with some success, I think.
What do you think?

First, I took 2 shots in natural light with a digital camera, from the front (or back side) and from the side.

With the microscope, it is impossible for me not to see the trichroism with lateral spots.

Then I put the crystal under the binocular:
1) With side spots.
2) Lower lighting like a thin section:
polarized or unanalyzed light.

With cross polarization, hues are enhanced.
Why ?

Crystal dimensions : weight = 1.0 g
base of the monoclinic prism ≈ 5 mm.
height ≈ 12 mm.

[cascaillou]

In a general way, if you can find the three colors of trichroism (blue / purple to magenta / yellowish to yellowish-brown or greenish), then you can say it is unheated.

[Roger Warin]

Thank you for this important remark, because there are cases where doubt remains.
One of my friends gave a talk at our club about a few gems including tanzanite.
If you wish, I can send you this article.
He is a gemologist (and professor) and his story is well documented.
I always appreciate your interventions on FMF.

[cascaillou]

Sure, I'd be interested in reading this article (sent you a pm)