Tanzanite (calcium aluminum silicate)

Color: Blue, violet, purple, orange, brown, colorless and pink.

Moh’s hardness: 6-6.5

Refractive index: 1.69-1.71

Critical angle: 35°

Specific gravity: 3.1-3.37

Cleavage: Perfect in 1 direction.

Fracture: Uneven, brittle

Dispersion: Moderately high (.030)

Heat sensitivity: Low

Pleochroism: Trichroic. Very 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 or brown to blue.

Cutting: Orient for best color and/or best yield.

Polishing: Cerium oxide or alumina on most common polishing laps.