Gray cobblestone backdrop is a crafted building block, similar to stone backdrop, only more cobbly. It's slightly darker than the gray cobblestone block it's designed to complement. You can find it in the Construction section of your inventory, where it appears as a blue-grey square with irregular divisions. This block is 1x1 square and has no collision.
How to Obtain
This block doesn't occur in naturally-generated structures such as bunkers and dungeons (January 2016), so the only ways to obtain it are either to craft it or to trade for it in a market world.
How to Use
Use this block as a backdrop when you want a slightly informal look. Gray cobblestone backdrop is not an ingredient in any crafting recipes. You can distress this block by hitting it with a hatchet. Note that mining distressed cobblestone backdrop returns stone rubble, so before distressing it, be sure you have it situated the way you want.
Cobblestone blocks and backdrops were released in late 2014/early 2015.