Azurite

Chemical Formula : Cu₃(CO₃)₂(OH)₂
Hardness : 3.5-4
Cleavage : Two, good
Luster : Vitreous
Color : Azure-blue to very dark blue crystals, lighter blue in massive specimens.
Streak : Light blue
Density : 3.77
Diagnostic Features and Trivia : Azurite is easily distinguished by its blue color and streak, its reaction to cold, dilute HCl, and is common association with Malachite.
Crystal System and Habit : Monoclinic; common crystals are tabular on {001} or {102}, short prismatic, massive or stalactitic.
Twinning : None
Polymorphs : None
Index of Refraction : alpha=1.730; beta=1.754-1.758; gamma=1.835-1.838