Correctly used, 'selenite' describes clear, colorless gypsum. The term is grossly overused, sometimes not even in reference to gypsum! This specimen, however, clearly deserves the moniker: it is a gypsum cleavage slab, half selenite (very water clear and very colorless), with the other half having a brown to green gray color due to abundant, fine grained iron oxide/hydroxide (?) inclusions. The division between the selenite and included zones is very sharp. An excellent example from this 2017 find which (to my knowledge) has not been repeated. Better in person than my historically lousy photos suggest.