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]