Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. (Min.) Eye agate. See under {Eye}.