Ferrocerium is around 4% magnesium.  Mostly it is a rare earth element stew blended into iron.  If it was just magnesium you would get a thermite reaction and could cast steel with the resulting fire.  back in the 1960s a thermite reaction with a ceramic crucible bottom drip was part of an Army armorers kit to sand cast steel parts for emergency repair parts.  It was a little stupid and short ranged in concept, but as it worked in an emergency to keep up a crew served weapon, I guess it wasn't really stupid, just a pain to do and file the part to fit.
blindhari
Sgt  Ranger