Tanzanite
Tanzanite (a variety of zoisite, Ca2Al3Si3O12(OH),
that contains vanadium in multiple oxidation states) shows
remarkable pleochroism (color change with viewing direction and
polarization of light)
When viewed in linerally polarized light (such as polarized sun
glasses) the color undergoes remarkable changes with the orientation of
the polarization.
Tanzanite crystal in the natural, unheated state
polarized vertically
unpolarized
polarized horizontally
The same crystal in side view
polarized
vertically
unpolarized
polarized horizontally