Marcasite

Chemical Formula : FeS₂
Hardness : 6-6.5
Cleavage : Two; distinct
Luster : Metallic
Color : Pale brass-yellow to nearly white
Streak : Greenish-black to brownish-black
Density : 4.89
Diagnostic Features and Trivia : Marcasite commonly forms what are termed cock's-comb structures that are expressed as rows of almost sharp points of its pyramidal crystals.
Crystal System and Habit : Orthorhombic; crystals common as tabular {010} or pyramidal.
Twinning : Extremely common on {100} in swallowtail contact twins; common {101}.
Polymorphs : Pyrite
Index of Refraction : Opaque