Tridymite

Chemical Formula : SiO₂
Hardness : 6.5-7
Cleavage : None, conchoidal fracture
Luster : Vitreous
Color : Colorless to white
Streak : White
Density : 2.26-2.27
Diagnostic Features and Trivia : Identified by the shape and size of crystals and habits. The crystals are actually inversion pseudomorphs after isometric high cristobalite.
Crystal System and Habit : Orthorhombic; small, pseudo hexagonal platy crystals and aggregates of wedge shaped twins
Twinning : See above
Polymorphs : Cristobalite and Quartz
Index of Refraction : omega=1.487; epsilon=1.484