Common Forms/Occurrences: Found in alabaster, satin spar, and selenite Useful Identification Test: Soft; Shows better cleavage than talc Chemical formula: CaSO4 ? 2H20 Crystal System: Tabular crystals, prisms, blades, or needles Colour: Colourless, white, or gray Streak: White Lustre: Nonmetallic (NM) Cleavage: Good Fracture: Conchoidal Hardness: 2 Hardness Search: two Specific Gravity: 2.3