This is not a scientific or historical fact but below are some observations I've made about tombstones from the 19th century. I've seen more marble tombstones than other materials. I've also noticed they aren't quite as decorative as they were in earlier centuries. However, they tend to have more decorative features than the 20th century.
Also, I've noted that they seem to have more information than previous centuries, so while they might not have as many images they do tend to tell you a bit more about the people.
Please note, this is an observation, it is historical fact. I've tried to find details about stones and the art of the tombstone cutters for the 19th century but have found little so far. If you have further information on the art of tombstones and the practices of the 19th century stone cutters, please share it.