This is how I draw rocks and soil in both Corel Painter and traditional media (mostly oil, acrylic, watercolor or oil pastel). There are no special brushes, filters, tricks, etc specific to Painter. Though I've never used photoshop, I assume these techniques would work there too. They're not difficult and take little drawing skill--just a lot of tedious practice. This tutorial assumes you already know how to handle basic light and shadow on an object; if not, there are other tutorials on dA for that.
You can see more of my references here [link]
but this is the first tutorial I've made and I would appreciate some feedback--is it useful, does the format work for you, is there some information you feel is missing?
Some examples of how I've used these techniques for rocks and soil in digital paintings: