Don't Leave Glass, Ceramic, Or Other Breakable Items On The Grave
They will break. Maybe not right away, if you are careful in setting it up, but they will break eventually. And someone (cough, cough . . . a grounds crew employee . . . cough) will have to clean it up.
If they don't spot the broken item right away, kids might pick it up and cut themselves or it could harm animals or become a projectile when the cemetery crew is mowing. Just don't do it.