Iolite

Color: Blue/purple and yellow with violet and/or gray undertones

Moh’s Hardness: 7-7.5

Refractive Index: 1.53-1.55

Critical Angle: 40.81°

Specific Gravity: 2.58-2.66

Cleavage: Distinct in 3 directions

Fracture: Uneven, conchoidal, brittle

Dispersion: Medium (.017)

Heat Sensitivity: Can be in my experience

Trichroic: Strong (yellow to dark blue-violet to pale-blue)

Birefringence (double refraction): Weak to medium (.008-.012)

Crystal Structure: Orthorhombic

Treatments: Generally none