Goethite

Chemical Formula : FeO(OH) or FeO(OH)-nH₂O
Hardness : 5-5.5
Cleavage : Two, one perfect, one good
Luster : Metallic to dull, fibrous specimens are silky, crystalline specimens are adamantine
Color : Blackish brown to yellow, or reddish brown to brownish yellow in massive specimens.
Streak : Brownish yellow to orange-yellow
Density : 3.3-4.3
Diagnostic Features and Trivia : Geothite is one of the lowest energy states for iron in the Earth's atmosphere, and so can often be found in pseudomorphs of crystalline or massivePyrite and Hematite.
Crystal System and Habit : Orthorhombic; uncommon crystals are prismatc to striated.
Twinning : None
Polymorphs : None
Index of Refraction : alpha=2.26-2.275; beta=2.393-2.409; gamma=2.398-2.515