Reaper Bones #77381 "Dragons Don't Share" diorama set -- plastic terrain base pieces painted up in acrylic paints, with some flocking, and a bit of epoxy putty for gap-filling.
The complete "Dragons Don't Share" set includes a dragon and several "adventurer" pieces, but here I just have the base terrain elements, used for tabletop gaming decor. The piece is pretty much painted up with an undercoat of mid-tone grey acrylic, and then I applied several mixes of a dark grey wash, occasionally mixing in splotches of watered-down dark grey-green or rusty brown, and alternating with light grey dry-brushing (once the washes had dried), to give the stone a varied and weathered look. The "earth/dirt" areas of the base (the smoother segments of plastic at the bottom and along the edges) were painted with a grey-brown acrylic mix, and then I sprinkled on "earth-tone" flocking before the paint dried -- essentially using the paint itself as "glue" (but without the resulting shine from using more conventional craft glue).
The really nice thing about this set is that it's cast in a durable, slightly flexible plastic, so it can weather a bit of abuse, and is far more suitable for transporting to conventions, etc., than more fragile structures made out of foam, casting concrete, papercraft, craft wood, etc. I do have a problem with paint wearing off fairly easily on the high points, but there's enough of a "pitted" texture that even then those points still look a little "dirty" and can pass for just another shade in the weathered stone. (Still, I expect I'll have to do a bit of touch-up work now and again.)