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.
Technically, being cut with a hammer and chisel the pictured tablet is probably ‘V’ cut. But notice that the original polish appears to be intact. Even if the letters were ‘skin cut’ with sandblast they would still be perfectly readable since the darkening caused by the polishing of the surface is what creates the contrast that makes the letter easily visible.
The polish on a stone has zero thickness it is simply an EXTREMELY flat uniform surface created by rubbing. Since the stone is still so dark looking where it has not been cut weathering in this case is VERY minimal. The stone, when weathered or smoothed but NOT polished is actually ash grey.
Jet Black granite is heaver and harder than the industry standard Barre Grey. Some of the denser examples ring like a bell when struck with a rubber mallet.