Gibbsite

Chemical Formula : Al(OH)₃
Hardness : 2.5-3.5
Cleavage : One; perfect
Luster : Pearly along cleavages, vitreous elsewhere.
Color : White
Streak : White
Density : 2.4
Diagnostic Features and Trivia : Rarely forms alone, and is one of the main constituent minerals in Bauxite.
Crystal System and Habit : Monoclinic; rare crystals are tabular {001} with {100} and {110}. More commonly as radiating spheroidal concretions.
Twinning : Twin axis [130] or twin plane {001}.
Polymorphs : None
Index of Refraction : alpha=1.568-1.58; beta=1.568-1.58; gamma=1.587-1.6