The Collaborative International Dictionary of English v.0.48:

Eyestone \Eye"stone`\ ([imac]"st[=o]n`), n.
   1. A small, lenticular, calcareous body, esp. an operculum of
      a small marine shell of the family Turbinid[ae], used to
      remove a foreign substance from the eye. It is put into
      the inner corner of the eye under the lid, and allowed to
      work its way out at the outer corner, bringing with it the
      substance.
      [1913 Webster]

   2. (Min.) Eye agate. See under Eye.
      [1913 Webster]