Most modern gravestones are made from granite. Older grave stones were often made from slate. Some from marble. If you walk through some of the colonial cemeteries, you will see that weathering has rendered most engravings unreadable. It's not just acid rain one needs to worry about, the erosion from constant freezing and thawing of absorbed water does a lot of damage.
There’s a few around here of marble. From 100 years ago. Hard to read. You have to put a piece of paper over it and rub a crayon over it. Still there though.