Andalusite Facet / Faceting Information
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.
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
What I prefer to polishing with: Alumina oxide and a tin lap, Spectra Untralaps work well too
Jeff’s Comments: This material is difficult to get clean and any decent size. The crystals are typicall long and narrow. I like this material, it cuts fairly easily and is quite different. It can be brittle, sharp corners may chip because of brittleness.