firestone

Noun

1. Iron pyrite, formerly used for striking fire.
2. A flint.
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.
   • On the descent the line is often in cuttings; some are high, such as at Scarcroft, where a cut through firestone and fireclay was necessary, and near Bardsey, where the line threads a deep tree-lined gorge. - 1961...