We don't have synonyms for eyestone yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are keystone, eyesore and merestone.
(n) :
A small, lenticular, calcareous body, especially an operculum of a small shell of the family Tubinidae, used to remove a foreign substance from the eye.
(n) :
A small, lenticular, calcareous body, especially an operculum of a small shell of the family Tubinidae, used to remove a foreign substance from the eye.