NOUN (2)

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;

The Collaborative International Dictionary of English v.0.48:

Firestone \Fire"stone`\ (?; 110), n. [AS. f[=y]rst[=a]n flint;
   f[=y]r fire + st[=a]n stone.]
   1. Iron pyrites, formerly used for striking fire; also, a
      flint.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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