gravestone vs stele

gravestone

noun
  • A stone slab set at the head of a grave. 

stele

noun
  • An upright (or formerly upright) slab containing engraved or painted decorations or inscriptions; a stela. 

  • The central core of a plant's root and stem system, especially including the vascular tissue and developed from the plerome. 

  • Any carved or engraved surface. 

How often have the words gravestone and stele occurred in a corpus of books? (source: Google Ngram Viewer )