The Collaborative International Dictionary of English v.0.48:

Gravestone \Grave"stone\, n.
   A stone laid over, or erected near, a grave, usually with an
   inscription, to preserve the memory of the dead; a tombstone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gravestone
    n 1: a stone that is used to mark a grave [syn: gravestone,
         headstone, tombstone]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "gravestone":
   arch, barrow, boundary stone, brass, bust, cairn, cenotaph, column,
   cromlech, cross, cup, cyclolith, dolmen, footstone, grave,
   headstone, hoarstone, inscription, ledger, marker, mausoleum,
   megalith, memento, memorial, memorial arch, memorial column,
   memorial statue, memorial stone, menhir, monolith, monument, mound,
   necrology, obelisk, obituary, pillar, plaque, prize, pyramid,
   reliquary, remembrance, ribbon, rostral column, shaft, shrine,
   stela, stone, stupa, tablet, testimonial, tomb, tombstone, tope,
   trophy