Pyrite

Chemical Formula : FeS₂
Hardness : 6-6.5
Cleavage : None, but conchoidal fracture
Luster : Metallic
Color : Brass-yellow
Streak : Black
Density : 4.9-5.2
Diagnostic Features and Trivia : Pyrite is paramagnetic. Also anyone who has ver played Dungeons and Dragons (or similar games) and used a twelve sided die knows exactly what a pyritohedron looks like. For those who haven't, it's simply a twelve sided figure whose sides are all pentagons.
Crystal System and Habit : Isometric; cubic, pyritohedral
Twinning : Twin axis [110], following the Iron Cross Law
Polymorphs : Marcasite
Index of Refraction : Opaque