A lot of the New England headstones were made out of sandstone. Easy to carve, easy to weather. Locally (I’m in CT) a number were made out of ‘Portland Brown’ a sandstone well known to people who live in NYC and quarried at a site about 10 miles from where I live. Sandstone is a sedimentary rock.
This is granite and granite is an igneous rock. One was produced by processes at the surface of the earth (sandstone), the other by volcanic processes deep in the earth (igneous).
You can easily cut sandstone with a cold chisel. But that means it is also easy to weather. If you get a small fissure in the headstone and you have quick freeze-thaw, as happens pretty often around here in Nov-Dec and late Feb-Mar, you get some physical weathering where the ice expands and makes a bigger fissure. And of course all that rain also affects the weathering (chemical), along with pollution either from lots of wood burning or coal or cars or Krakatoa blowing its top.
But granite and most igneous rocks are tough to physically weather (or physically inscribe), and chemical weathering, while it happens, does so on a time scale we humans would have trouble noticing.
Thank you for your full, knowledgable and interesting explanation. :-)
The Portland brown is good looking in construction but useless as a lasting memorial. I think it needs some kind of surface coat that can come off in a century or so. The entire gate of The Greenwood Cemetery in Brooklyn New York was made with that or something enough like it to not matter. It is periodically resurfaced (or something) to keep it from crumbling.
There are a fair amount of tombstones made with it here and what ever it is that the letters are cut into shears off in slabs in spots. The stone underneath is a lighter brown. I’ve never been quite sure what it (the surface)is.