Topaz

Chemical Formula : Al₂SiO(F,OH)
Hardness : 8
Cleavage : One, perfect
Luster : Vitreous
Color : Colorless to white, but also pale blue, yellow, yellow-brown.
Streak : White
Density : 3.5-3.6
Diagnostic Features and Trivia : Topaz is the definition of hardness 8 on Moh's scale.
Crystal System and Habit : Orthorhombic; common crystals are large, well developed prisms, typically terminating in a wedge.
Twinning : None
Polymorphs : None
Index of Refraction : alpha=1.606-1.630; beta=1.609-1.631; gamma=1.616-1.638