First, I cut a piece of rigid foam or use a piece of an old mannikin.
Next, I sand or scrape it to the shape I want, making sure to sand off all of the slick areas where mold release was used.
Next, I mix 50/50 Durabond powdered drywall plaster and Vermiculite ( I get it at plant stores or nurseries)
Next, I cover the entire foam piece with it, except the bottom, and let it set up for a few minutes.
Once it has set and firmed up a little, I use a damp paper towel and smooth down some of the pointy looking areas.
After this has set up ( about 20 minutes ), I mix about 5 parts water to 1 part powdered Tempra paint.
Using a 1 “ paint brush or a Tempra brush, I dab on the paint making sure it gets down in the cracks and crevices of the rock.
Next I dab the rock with a dry paper towel until I achieve the look that I am after. I used Black Tempra on this rock, but you can use any color you want, and even mix them.
Very simple and saves a lot of money if you need to add some rocks to your habitat base! I attach them with hot glue.