Idocrase (calcium aluminum silicate)
Color: Olive-green, to yellow-brown.
Moh’s hardness: 6.5
Refractive index: 1.7-1.721
Critical angle: 35.5°
Specific gravity: 3.32-3.42
Fracture: Uneven, splintery
Dispersion: Moderately high (.019)
Heat sensitivity: Low
Pleochroism: Dichroic – weak (lighter to darker).
Birefringence (double refraction): Weak (.005)
Crystal structure: Tetragonal
Cutting: Cuts with relative ease.
Polishing: Alumina on standard polishing laps.
Comments: Flawless stones are very rare.