Curiously enough, there’s no selenium in selenite — it’s a crystalline form of gypsum, and gets its name from its transparency and lack of color. The word comes from Greek via Latin, and literally means “moonstone.” The thought was, in ancient times, that certain transparent crystals waxed and waned with the moon.
In modern times, selenite is primarily valuable in the form of collector specimens like this — and it’s both water-soluble and very soft (you can scratch it with a fingernail), so needs to stay behind glass. This sample came from the Naica mine in Chihuahua, Mexico — famed for selenite crystals that in some cases grew to 15 meters (50 feet) in length. Gypsum per se is a common sulfate mineral found across the globe, and as a result is fairly cheap in bulk, most commonly used in plaster and drywall.