Source : Webster's Revised Unabridged Dictionary (1913)

Firestone \Fire"stone`\ (?; 110), n. [AS. f?rst[=a]n flint; f?r
   fire + st[=a]n stone.]
   1. Iron pyrites, formerly used for striking fire; also, a
      flint.

   2. A stone which will bear the heat of a furnace without
      injury; -- especially applied to the sandstone at the top
      of the upper greensand in the south of England, used for
      lining kilns and furnaces. --Ure.

Source : WordNet®

firestone
     n 1: a piece of flint that is struck to light a fire
     2: a sandstone that withstands intense heat; used to line
        fireplaces and furnaces and kilns