Andalusite (aluminum silicate)
Color: brown, green, pink, violet, red.
Moh’s hardness: 7-7˝
Refractive index: 1.641-1.648
Critical angle: 37.5°
Specific gravity: 3.12-3.18
Cleavage: Imperfect, but distinct in one direction.
Fracture: Uneven, brittle.
Dispersion: Medium (.016)
Heat sensitivity: Low
Pleochroism: Trichroic – Strong; reddish to yellow to olive green.
Birefringence (double refraction): Medium (.007)
Crystal structure: Orthorhombic
Cutting: Cut on a medium lap. Orient for desired color. Sharp corners may chip because of brittleness. Stay clear of the cleavage plane.
Polishing: Alumina or tin oxide on tin or tin/lead lap at normal speed.