The Collaborative International Dictionary of English v.0.48:

Magnetite \Mag"net*ite\, n. (Min.)
   An oxide of iron (Fe3O4) occurring in isometric crystals,
   also massive, of a black color and metallic luster. It is
   readily attracted by a magnet and sometimes possesses
   polarity, being then called loadstone. It is an important
   iron ore. Called also magnetic iron.
   [1913 Webster]

WordNet (r) 3.0 (2006):

magnetite
    n 1: an oxide of iron that is strongly attracted by magnets
         [syn: magnetite, magnetic iron-ore]