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Petalite (Lithium Aluminum silicate)
Color: colorless, light greenish or pink.
Moh’s hardness: 6-6.5
Refractive index: 1.50-1.52
Critical angle: 41.5°
Specific gravity: 2.40
Cleavage: Perfect in one direction.
Fracture: Conchoidal, brittle.
Dispersion: Medium (.014)
Heat sensitivity: Low
Birefringence (double refraction): Medium (.012-.016)
Crystal structure: Monoclinic
Cutting: Use medium and fine laps.
Polishing: Use tin/lead lap with alumina or cerium oxide; or aluminum oxide Ultralaps. Run polishing laps at slow speed.
Comments: The cleavage plane usually doesn't cause any problems, but will only polish to a pearly luster. Petalite cuts into bright stones with a vitreous luster.