Grossular garnet luminescence

Note added on 24 May 2017: I have concluded now that the 'chrysoberyl' must actually be a green grossular garnet ( Ca3Al2(SiO4)3 ) similar to tsavorite. The single refractive index of the crystal of 1.745 is consistent with this conclusion. The crystal does, however, show a marked brown/green colour change under tungsten and daylight illumination.

It is divided into green and light brown regions, both of which fluoresce red when illuminated with a 404nm laser. Compared with the chromium containing stones, the fluorescence is very weak.

The complex of emission lines and bands between 680 and 800nm is thought to be due to the transition metal ion vanadium (V^2+) which has a similar structure (is isoelectronic with) Cr^3+.
The luminescence is similar to the red emission in zoisite (tanzanite) which is also thought to be primarily due to vanadium. For this reason, I show the zoisite spectrum here for comparison.

The left-hand image is a white light exposure showing the green and brown colour. The right-hand image shows the red luminescence in the path of the blue laser beam photographed through a yellow filter (which greatly reduces but does not completely remove the laser light).

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