Zoisite (tanzanite)

The polarised transmission and fluorescence of a cut specimen of the gemstone tanzanite. The mineral (Epidote) is Ca_2 Al Al_2 O(OH)(Si_2 O_7)(SiO_4). The stone is pleochroic with the colours light blue, dark blue and purple.

The two transmission spectra here are taken through one direction with orthogonal linear polarisations: the colours appear light blue (blue line) and pink (red line). Note the unusual and characteristic break in the (red) spectrum at 680nm.

The fluorescence, excited with a 404nm laser and observed with a yellow filter (black line), shows emission due to samarium (Sm^3+, between 500 and 670nm) and vanadium (V^2+, between 670 and 800nm) and possibly other rare earths.

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