No, its not gypsum in its natural state as mined, but it could be a clunker for some other industrial process. They produce cement by super heating limestone and other materials and then grinding the resulting clunkers to make cement. They use gypsum for a bunch of things including smelting steel. These could be clunkers that just haven't been ground up yet. Again, I have no idea what this guys is selling but if they came out of Nevada its a possibility.
Its strange that he can't tell you what it is though....maybe ask him where he got them from?