Achroite Tourmaline

Color: (Greek - without color) colorless or nearly so, fairly rare

Moh’s Hardness: 7-7.5

Refractive Index: 1.616-1.652

Critical Angle: 38.23°

Specific Gravity: 3.02-3.26

Cleavage: None

Fracture: Uneven, small conchoidal

Dispersion: Medium (.017)

Heat Sensitivity: Can be, but usually no problems

Dichroic: Strong (many color variations), on colorless material obviously not generally present

Birefringence (double refraction): Medium (.014-.024)

Crystal structure: Hexagonal

Treatments: Heat treating to lighten colors is common and sometimes material will go clear (Achroite).