Augite

Chemical Formula : (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)₂O₆
Hardness : 6
Cleavage : Two, good
Luster : Vitreous, sometimes mildly pearly
Color : Bright to dark green, grayish green, brown, black.
Streak : White
Density : 3.25-3.55
Diagnostic Features and Trivia : Augite is a memeber of the pyroxene group, and is easily noted by its cleavage, color, and luster.
Crystal System and Habit : Monoclinic; common crystals as short prisms ususally nearly square in cross section.
Twinning : Twin plane {100}
Polymorphs : None
Index of Refraction : alpha=1.671-1.735; beta=1.672-1.741; gamma=1.703-1.761