Shape. Well, form, really. Shape is the two dimensional aspect of what we see and Form is the three dimensional aspect. Cezanne said there are four basic forms: the cone, the cube, the cylinder, and the sphere.
Rocks are "formed" chunks of solidified magma that have been chipped apart and worn away to varying degrees. The more the rocks are weathered and tumbled by wind and wave motion - and depending on the softness of the materials they are made of - the smoother the surface and the more rounded the form they have.
To draw realistic looking rocks requires paying attention to how the 3D form of the rock produces highlights and shadows, and then to rendering them as lighter and darker 2D areas flat on the paper.