CaSO₄

Non-silicate

Crystal system: orthorhombic (+)

Colour in hand specimen: white or colourless when pure 
Cleavage: two at 90° 
Hardness: 3-3.5

Colour in plane polarised light: colourless 
Pleochroism: none 
Relief: low 
Birefringence: ~0.04; 3^rd order blue 
Extinction: straight 
Twinning: simple or repeated

CaSO₄∙2H₂0

Non-silicate

Crystal system: monoclinic (+)

Colour in hand specimen: usually white or colourless
Cleavage: rhombohedral, one perfect, two distinct
Hardness: 2

M530 Gypsum 
Shotover, Oxford 
Single euhedral crystal 
Colourless, transparent 
Hardness 2; Soft – scratched with your fingernail

M531 Massive gypsum 
Nottingham 
Fibrous habit – many fine crystals 
White-cream colour 
Translucent 
Pearly lustre