Cristobalite

Chemical Formula : SiO₂
Hardness : 6.5
Cleavage : None, conchoidal fractue
Luster : Vitreous
Color : White
Streak : White
Density : 2.32-2.34
Diagnostic Features and Trivia : Easliy identified by the shape and size of crystals and habit. These crystals are actually inversion pseudomorphs after isometric high-cristobalite.
Crystal System and Habit : Tetragonal; uncommon crystals are small and octahedral.
Twinning : {111}
Polymorphs : Quartz and Tridymite
Index of Refraction : omega=1.487; epsilon=1.484