The Collaborative International Dictionary of English v.0.48:

Ironstone \I"ron*stone`\,
   1. n. A hard, earthy ore of iron.
      [1913 Webster]

   2. ironstone china.
      [PJC]

   Clay ironstone. See under Clay.

   Ironstone china, a hard white pottery, first made in
      England during the 18th century.
      [1913 Webster]