Magnetite
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Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe²⁺Fe3+2O₄. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. Wikipedia
Crystal system: Isometric
Cleavage: Indistinct, parting on, very good
Color: Black, gray with brownish tint in reflected sun
Crystal habit: Octahedral, fine granular to massive
Diaphaneity: Opaque
Fracture: Uneven
Lodestone: Magnetic with definite north and south poles