Sharp broken rocks, made to look like larger rocks shattered over time. They are arranged on the ground plane exactly as pictured here. Tuck them into your ground for natural looking scattered stones. Scale them up to make sharp boulders, or very small for tiny specs of stone.
However you use them, these rocks will remain a valuable resource you can pull out to detail many projects. I always tuck a few in the background of outdoor scenes to quickly break up large areas of ground and add that natural variation to the environment.