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Kyanite (aluminum silicate)
Color: Colorless, blue, gray.
Moh’s hardness: 4-4.5 along axis, 6-7 along plane.
Refractive index: 1.715-1.732
Critical angle: 35.5°
Specific gravity: 3.5-3.69
Cleavage: Very perfect in 2 directions.
Fracture: Splintery, brittle.
Dispersion: Moderately high (.02)
Heat sensitivity: Low
Pleochroism: Trichroic – definite (blue to pale or colorless).
Birefringence (double refraction): Medium (.017)
Crystal structure: Triclinic
Cutting: Sawing must be done with the hard face placed against the table. Sharp corners tend to fray. Stay clear of cleavage planes. Cutting should be done in the direction which is parallel to the strong cleavage plane.
Polishing: Alumina on tin/lead at slow speed.