eyestone

 A small, lenticular, calcareous body, esp. an operculum of a small marine shell of the family Turbinidæ, 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]

 Eye agate. See under Eye.  [1913 Webster]