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[Roger Warin]

Hello,
A tourmaline section of Mont Ibity (Madagascar), this is not exceptional.
But this one is seen by transparency with a binocular (panoramic stacking).
I actually took this picture to "prove" that elbaite is uniaxial (negative).
We obtain when the mineral is seen according to the c axis (elongation), a Maltese cross encompassing a nice set of interferences (rainbows).
This experience is certainly useless, but it is pleasant for me to better understand what I learned 70 years ago!

The photo shows that the cross is not centered but is slightly moved to the right. The cause of this discrepancy is that the Malagasy cutter did not cut the tourmaline perpendicular to its c axis. We can also see it on the photo on which we see a draft of the prism on the left.