Definition of "firestone"

firestone

noun
countable and uncountable, plural firestones
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.
  Quotations
  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 February, D. Bertram, “The lines to Wetherby and their traffic”, in Trains Illustrated, page 101