A rock is the only solid object on the surface map. It can provide protection since spider "pathfinding" does not allow it to go around the rock if you are on the other side.
There are four types of rock, each with different textures, details, and (like mcnuggets) names for each shape. The four in game rocks are labeled as triangle, rhombus, skipper, and curvy.
The hitbox of the rock is smaller than the rock itself. This allows the player to be able to move over the edges of the rock (usually the narrow ends) and even set up a colony under the rock. A colony made under the rock will remain hidden until it has grown to a considerable size.