Definition of "selenite"

I followed them to my study, and found lying on my writing-table still, with the selenite paper-weight upon it, the sheet of work I had left on the afternoon of the opening of the cylinder.

1898, H.G. Wells, The War of the Worlds, London: William Heinemann, page 295

Glass windows replaced the old tiny windows of selenite (crystallized gypsum), and stovepipe replaced adobe chimneys.

1994, Stewart Brand, How Buildings Learn: What Happens After They’re Built, New York: Viking, page 146