Malachite
Chemical Formula : Cu2(CO3)(OH)2
Hardness : 3.5-4
Cleavage : Two, one perfect, one distinct
Luster : Silky, velvety, or dull
Color : Bright green
Streak : Pale green
Density : 4.05, but dropping to 3.6 for massive specimens.
Diagnostic Features and Trivia : Malachite is easily distinguished by its green color and streak, its reaction to cold, dilute HCl, its common botroydal habit and is common association with Azurite.
Crystal System and Habit : Monoclinic; rare crystals are prismatic to fine acicular, and are grouped in tufted rosettes. Commonly malachite forms mammilary or botroydal encrustations that exhibited banding in cross section.
Twinning : Twin plane {100}
Polymorphs : None
Index of Refraction : alpha=1.655; beta=1.875; gamma=1.909