Compressed Granite marks the map boundaries. It is found both at the top and bottom of the map, surrounding the destructible terrain around the procedurally generated tunnels and caverns. There is a thin layer of another sturdy material named Bed Rock.
Its surface is smooth, shiny and completely black. It is an incredibly tough material, requiring five hits with your Pickaxe before it breaks, and you will mine out a much smaller area than other types of Common Rock.
Compressed Granite and Bed Rock cannot be drilled, it is assumed that the Driller's Reinforced Power Drills are not powerful enough to break it apart, while larger drills such as the one on the Drop Pod and Resupply Pod can.