That was A19. I'll go check A18, jussa sec.
Aaaaaand we're back. Dang A18 takes forever to load. Okay it looks like Stone is a special case (though I did not yet test all terrain blocks). Forest Ground and Asphalt both take on whatever the original surface texture was. Stone just looks like stone. I tested this in both A18 and A19.
Top blocks are Forest Ground, bottom blocks are Stone. This is on a concrete platform 4 blocks above the surface. I also tested beneath the surface and the same behavior holds true.