\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.