Gypsum is calcium sulphate dihydrate, CaSO4 . 2H2O This variety of rosette crystal habit is commonly referred to as a "desert rose". This is because it is found in arid desert conditions when shallow salt basins dry up. As the basin dries up the gypsum is forced to precipitate, and as it does so it incorprates grains of sand in its growth. This particular specimen is from Morocco, but similar specimens are found in most of the worlds deserts.