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]

The Collaborative International Dictionary of English v.0.48:

Chert \Chert\ (ch[~e]rt), n. [Ir. ceart stone, perh. akin to E.
   crag.] (Min.)
   An impure, massive, flintlike quartz or hornstone, of a dull
   color.
   [1913 Webster]

WordNet (r) 3.0 (2006):

chert
    n 1: variety of silica containing microcrystalline quartz