Gypsum
CaSO₄·2H₂O · Monoclinic · also called selenite, satin spar, alabaster, desert rose
Gypsum is the standard for hardness 2 and the mineral in plaster and drywall. Selenite (clear crystals), satin spar (silky fibrous), and alabaster (fine-grained massive) are all the same mineral. If a clear, glassy-looking crystal yields to your fingernail, gypsum should be your first guess.
Properties
- Mohs hardness
- 2
- Streak
- white
- Lustre
- vitreous, pearly, silky
- Colour
- Usually colourless or white; sand inclusions give desert roses their brown colour.
- Cleavage
- Perfect in one direction — selenite peels into thin, flexible, transparent sheets.
- Fracture
- Splintery to conchoidal
- Specific gravity
- 2.3–2.4 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm gypsum
- A fingernail scratches it. Very few other transparent minerals are this soft.
- Thin sheets bend without springing back — they are flexible but not elastic.
- Does not react to acid, which separates it from the carbonates it often accompanies.
What gypsum gets confused with
Each of these is settled by one test. Run that test first.
Calcite fizzes in acid and resists a fingernail. Gypsum does neither.
Muscovite sheets spring back when bent; gypsum sheets stay bent.
Where it is found
Evaporite beds, desert playas, clay-rich soils, and as crystals lining cavities in shale.
Is it worth anything?
Very common and inexpensive. Large intact selenite crystals and well-formed desert roses sell modestly.