Gypsum
Images (c) 2000, 2003, 2004 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Gypsum is a soft mineral, hydrous calcium sulfate. Gypsum is the standard for hardness degree 2 on the Mohs mineral hardness scale. Your fingernail will scratch this mineral—that's the simplest way to identify gypsum. The clear variety that makes up this wedge is called selenite after the pearly luster of its cleavage faces, likened to moonlight.
Gypsum also forms concretions of selenite blades called desert roses or sand roses, growing in sediments that are subjected to concentrated brines. The crystals grow from a central point, and the roses emerge when the matrix weathers away. They don't last long at the surface, just a few years, unless someone collects them. Here's another one.
Besides gypsum, barite, celestite and calcite also form roses. See other common mineral shapes in the Mineral Habits Gallery
Most gypsum occurs not in crystals but in massive chalky beds of rock gypsum. It's mined for the manufacture of plaster, and household wallboard is filled with gypsum.
Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society
