Actually the thing is pure zinc sulfide doesn’t show flourescence,but it fluoresce due to presence of cadmium and silver impurities. These impurities are mixed into zinc sulfide lattice.

Why is zinc sulfide fluorescent?

The zinc sulphide glowed because the decay products from the radium excited the electrons, as explained above.

What color is zinc sulfide?

CAS Reg. No.1314-98-3SciFinder nomenclatureZinc sulfide (ZnS)Empirical formulaSZnMolar mass97.47 g/molAppearanceWhite, grayish-white, or yellow crystals or powder