Grab a free set of 6 rock IMM brushes for ZBrush 4R8 from Glenn Patterson.
Set of 6 rock Insert Multi Mesh brushes for ZBrush 4R8. Rocks created by using Scan Data, Decimated, and sliced in half.
By downloading the rock IMM brush you agree that any image or video produced (personal or commercial) using the brush is yours. The rock brush and models themselves may not be redistributed or sold.
You can find more details on the pack here.