There are a few ways I make rocks.
First is using real rocks.
Second is using Sculpey (polymer clay). I prefer plain, white, original Sculpey to any other variety of polymer clay for making bases.
Anyways, using Sculpey, I take a blob about the size I want my rock, and press actual rocks into it to get a rock texture. You can do this with shale (my neighbor has it in his landscaping and a few pieces made their way into my house), or any type of rock. I have a box of rocks with different textures that I use depending on what I want the rocks to look like.
Then you bake the Sculpey as directed on the package.
The second way I like to use Sculpey is to roll it out in a sheet (using the clay extruder/pasta maker is awesome for this) on a ceramic tile, stick it in the oven and bake as directed, and then break the sheet apart. I'll paint it as rock or I'll carve into it with a scribing tool or wood carving tool and make it look like tiles.
It sounds more complicated than it actually is, once you do it.
Third way I do rock is to use cork. I've found, in my scrounging, that there are different textures of cork and I've used as many as I can find.
All these techniques for making rocks go hand in hand with using sand and gravel to fill in the cracks, gaps, holes or whatever, and the different textures make the bases more interesting.
Also, combining the cork and Sculpey produces some very fun results. And then sand and gravel. Then prime and paint. Done!