Hauynite (Sodium aluminum calcium sulfo-silicate)
Color: Blue to greenish blue.
Moh’s hardness: 5.5-6
Refractive index: 1.49
Critical angle: 42°
Specific gravity: 2.4-2.5
Cleavage: Moderate in six directions.
Heat sensitivity: Low
Crystal structure: Isometric
Cutting: Cutting is fairly straightforward.
Polishing: Polishes easily with alumina tin/lead or chrome oxide Ultralaps.
Comments: Rarely found in anything but very small stones. Hauynite is a constituent of lapis lazuli.