Andalusite

 

Color: brown, green, pink, violet, red.

Moh’s Hardness: 7-7½

Refractive Index: 1.641-1.648

Critical Angle: 37.57°

Specific Gravity: 3.12-3.18

Cleavage: Imperfect, but distinct in one direction.

Fracture: Uneven, brittle.

Dispersion: Medium (.016)

Heat Sensitivity: Yes, moderate in my experience

Pleochroism: Trichroic - Strong; reddish to yellow to olive green

Birefringence (double refraction): Medium (.007)

Crystal structure: Orthorhombic

 

Treatments: Generally none