firestone

/F AY1 R S T OW0 N/
noun
  1. 1

    Iron pyrites, formerly used for striking fire.

  2. 2

    A flint.

  3. 3

    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.

Source: https://en.wiktionary.org/wiki/firestone