It’s rectangles of 2-inch solid foam insulation glued onto a plywood frame. We carved the stones with knives and smoothed the edges with the side of a soldering iron. Then we pressed the surface of a big rock all over it for texture, and painted it with mod podge to seal it. Next will paint it gray and do some dry-brushing with black, brown and rust to give the rocks some variety. The “mantle” on top is an already varnished pine board salvaged from built-in bunk beds I made for my kids back in the day.