Color: Blue and yellow with violet and/or
Moh’s hardness: 7-7.5
Refractive index: 1.53-1.55
Critical angle: 40°
Specific gravity: 2.58-2.66
Cleavage: Distinct in 3 directions.
Fracture: Uneven, conchoidal, brittle.
Dispersion: Medium (.017)
Heat sensitivity: Low
Pleochroism: Trichroic – very strong
(yellow to dark blue-violet to pale-blue).
Birefringence (double refraction): Weak to
Crystal structure: Orthorhombic
Cutting: Orient for color.
Large stones are
usually too dark. Iolite has fairly strong cleavage in one direction and
fractures along this plane are common. Keep this cleavage 7 to 10 degrees off of
the plane of the table. The other cleavage planes usually don’t create
problems, but occasionally cause difficulties in polishing.
Polishing: Cerium oxide or alumina on common