From Earl Verbeek, Franklin Mineral Museum via Facebook: "At least two different activators. The orange emission is due to manganese. The blue emission has been ascribed to silver, but I don't know of any proof of that. A bit of silver impurity can cause blue emission in ZnS phosphors, but so can other activators, so I don't think that question has been settled yet for Sterling Hill sphalerites. In any event, the combination of blue and orange emission is what your eyes see as pink. I once measured the emission spectra of a SH sphalerite like yours and documented that the orange emission strengthened and the blue emission weakened as I changed the wavelength of the UV light. You are seeing that as well."