Color: Blue and yellow with violet and/or gray undertones.

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 medium (.008-.012)

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 polishing laps.