Go to an office supply store, or that department at a walmart.
Get corkboard, they sell it for cheap in packs. Use snips to chop it up, if you want it thicker, just glue pieces together.
Once you have it looking kinda like you want it, paint it black. Be sure to get in every nook and cranny.
Next step, drybrush with a dark grey. Then again, a lighter coat wit a lighter grey, then just hit the edges with white.
You can play with it to get the shade you want. Also, for wood pieces...well, cork IS a kind of wood and you have those snips...
Is how I did the rocks in this...