'Hornstone' definitions:

Definition of 'hornstone'

(from WordNet)
noun
A fine-grained metamorphic rock formed by the action of heat on clay rocks [syn: hornfels, hornstone]

Definition of 'Hornstone'

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