One way to figure out the era your ancestor was buried is to examine the material from which the tombstone is made.
1796-1830 - A stone made of slate or common fieldstone.
1830-1849 - A stone that is flat topped hard marble.
1845-1868 - A stone that is round or pointed soft marble with cursive inscriptions.
1850 - now - Masonic four-sided stones.
1860-1900 - A stone with pylons, columns and all exotic-style monuments.
1870-1900 - Zinc monuments.
1900 - now - Granite stones.