A selection of gypsum specimens from a variety of caves as well as CaSO4 synthesized in the laboratory emit both a green and yellow candoluminescence when excited by a hydrogen diffusion flame. The green emission is attributed to dehydration of gypsum to bassanite and the yellow emission appears upon further dehydration to anhydrite. The source of the luminescence is ascribed to minor concentrations of Mn2+ in the gypsum.
Sweet, John R.; John W. Hess; and William B. White.
Candoluminescence of cave gypsum.
International Journal of Speleology,
Available at: http://scholarcommons.usf.edu/ijs/vol39/iss1/3