Quartz

Chemical Formula : SiO₂
Hardness : 7
Cleavage : None, but conchoidal fracture
Luster : Vitreous
Color : Clear to white when pure, but any other color is possible with impurities.
Streak : White
Density : 2.65 macrocrystalline, ~2.6 cryptocrystalline
Diagnostic Features and Trivia : .
Crystal System and Habit : Hexagonal (trigonal); .
Twinning : Very common on {101} as simple contact, repeated contact, or cyclic with six to eight individuals.
Polymorphs : Tridymite and Cristobalite
Index of Refraction : omega=2.065-2.616; epsilon=2.89-2.903