Hornblende
Chemical Formula : Ca(Mg,Fe)4Al(Si7Al)O22(OH,F)2
Hardness : 6
Cleavage : Two, perfect
Luster : Vitreous
Color : Dark green, dark brown, black.
Streak : White
Density : 3.0-3.4
Diagnostic Features and Trivia : Hornblende is a memeber of the amphibole group, and is readily identified by its dark color and perfect cleavage.
Crystal System and Habit : Monoclinic; common crystals are prismatic with hexagonal cross section.
Twinning : Twin plane {100}
Polymorphs : None
Index of Refraction : alpha=1.61-1.70; beta=1.62-1.70; gamma=1.63-1.71