Source : Webster's Revised Unabridged Dictionary (1913)

Magnesite \Mag"ne*site\, n. [Cf. F. magn['e]site.] (Min.)
   Native magnesium carbonate occurring in white compact or
   granular masses, and also in rhombohedral crystals.

Source : WordNet®

magnesite
     n : a black magnetic mineral consisting of magnesium carbonate;
         a source of magnesium