Epidote (calcium aluminum iron silicate)
Color: Pistachio-green, yellow-brown and blackish-brown.
Moh’s hardness: 6-7
Refractive index: 1.733-1.768
Critical angle: 35°
Specific gravity: 3.25-3.5
Cleavage: Perfect in two directions.
Fracture: Uneven, brittle.
Dispersion: Moderately high (.03)
Heat sensitivity: Moderate
Pleochroism: Trichroic – Strong (green to brown to yellow).
Birefringence (double refraction): Strong (.04)
Crystal structure: Monoclinic
Cutting: Cutting is fairly straightforward.
Polishing: Alumina on standard polishing laps.
Comments: Cleavage tends to separate with heat during dopping. The twinning planes are even more susceptible to heat.