Source : Webster's Revised Unabridged Dictionary (1913)

Hornstone \Horn"stone`\, n. (Min.)
   A siliceous stone, a variety of quartz, closely resembling
   flint, but more brittle; -- called also {chert}.

Source : WordNet®

hornstone
     n : a fine-grained metamorphic rock formed by the action of heat
         on clay rocks [syn: {hornfels}]