Selenite and gypsum: soft enough to scratch with a fingernail

Gypsum is a 2 on the Mohs scale, the reference mineral for that hardness point, meaning a fingernail will genuinely scratch it, a fast and reliable field test. Selenite is simply the clear, crystalline variety of gypsum, and thin sheets of it are flexible enough to bend slightly without breaking, a real and distinctive property for a mineral this soft. Gypsum is also the mineral responsible for desert rose formations, rosette clusters of flattened crystals that grow in arid, evaporite rich sediment, though barite forms very similar looking desert rose clusters through a different process, so the two are easy to confuse by appearance alone.