Overgrown Granite/Diorite/Andesite have a stone footstep sound instead of grass sound.

As well for some reason i can use bone meal on them, but nothing grows.
Placing another block on top of overgrown one does not remove grass from it, which is kinda strange as well.

And, i guess, regolith might be used as farmland, since

Regolith - all surface loose formations representing the upper layers of the earth's surface: humus soils, alluvium, weathering products of rocks, what are laterite, eolian deposits, slope talus, glacial deposits, etc.