I've been experimenting with different workflows natural assets such as rock and wood. Here's an update of one of the better workflows I found, which consisted of modelling a rough shape in Maya or your basic 3D package. Then jumping into ZBrush, where most of the work was done, and sculpting at different sub-D levels some different detail passes. Then I decimated my final sculpt and using the newly created low-poly version to bake out a normal map for the overall shape. I then used various texturing techniques in Photoshop and created a some maps for it, making sure to also keep it tileable. The same was done for the surface detail normals I created in PS as well using nDo on the diffuse map. This results in a nice bumpy rock texture, that looks particularly good on the rock I have created, but can also be applied to other rocks seamlessly in UDK.
I created a material in UDK that is aware of the scale of the object it is applied to, which is great for environment artists. It allows you to scale the asset up and down, and the material automatically updates it's own scale within the viewport. This is why it's important to make sure the texture was tileable.
Below is an example of exactly the same rock, scaled and rotated a few times, to give the illusion of a large rock formation. All of this was done with just one rock and one texture, and the assembling of these rocks below took seconds due to efficient workflow practises.