Kornerupine (magnesium aluminum silicate)
Color: green, green-brown and colorless.
Moh’s hardness: 6.5-7.
Refractive index: 1.665-1.682
Critical angle: 37°
Specific gravity: 3.28-3.35
Cleavage: Distinct in 2 directions.
Fracture: Conchoidal, somewhat brittle.
Dispersion: Moderately high (.018)
Heat sensitivity: Low
Pleochroism: Dichroic – Strong (green, yellow and brown).
Birefringence (double refraction): Medium (.013)
Crystal structure: Orthorhombic.
Cutting: Cleavage may be a problem when roughing in.
Polishing: Alumina on tin or tin/lead.