The Collaborative International Dictionary of English v.0.48:

Hornstone \Horn"stone`\, n. (Min.)
   A siliceous stone, a variety of quartz, closely resembling
   flint, but more brittle; -- called also chert.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hornstone
    n 1: a fine-grained metamorphic rock formed by the action of
         heat on clay rocks [syn: hornfels, hornstone]