Tanzanite

Color: Blue, violet, purple, orange, brown, colorless and pink, also some collector colors

Moh’s Hardness: 6-6.5

Refractive Index: 1.691-1.70

Critical Angle: 36.28°

Specific Gravity: 3.10-3.37

Cleavage: Perfect in 1 direction

Fracture: Uneven, brittle

Dispersion: Moderately high (.030)

Heat Sensitivity: Moderate

Pleochroism: Trichroic - strong. Blue to purple to orange or brown

Birefringence (double refraction): Weak (.006-.009)

Crystal Structure: Orthorhombic

Treatments: Usually heat-treated to change color from orange/brown to blue/purple