Magnetite

Chemical Formula : Fe₃O₄
Hardness : 5.5-6.5
Cleavage : None, but parting is common along twin planes
Luster : Splendent to dull-metallic
Color : Black
Streak : black
Density : 5.17
Diagnostic Features and Trivia : As the name suggests, magnetite is magnetic. This is because it contains both ferric(Fe⁺³) and ferrous(Fe⁺²) oxides.
Crystal System and Habit : Isometric; commonly octahedral
Twinning : Common on {111} following the Spinel Law
Polymorphs : None
Index of Refraction : Opaque except in thin splinters; 2.42