They are large, round rocks, about the size of the Pygmy.
Boulders can crash into Pygmies, and send them hurdling down the mountain. The boulders will roll down the mountain, attempting the crush the pygmy, and will go down every-which-way it can. The boulder and the beetle are the ape's two methods of killing the pygmy. The boulders can be destroyed with the pick axe, but, in later levels, the area is too large that the pick axe normally isn't positioned right. Every jump the player makes over a boulder gives them 100 points. Konkey has a never ending supply of boulders at his disposal.