Rocks can be found at various locations throughout the map. Not all rocks can be harvested. Some islands do not have any harvestable rocks while others have many.
A rock can be harvested by using a pick axe or a chipped stone (starting weapon/tool) to get stone which is used in many craftable and build items. The total amount of stone yielded depends on the size of the rock with bigger rocks giving more stone than smaller ones.
Once harvested the rock will break into fragments then disappear after a few seconds. Another rock will spawn in a different random location.
Note: a fragmented rock does not damage the player.