Alternative base rocks
I have used both lava (rarely) and tufa rock in my systems with good success. I have thoroughly rinsed/washed both before use. Over time, they became so encrusted that it was difficult to tell that they were not reef rock.
I have also used homemade "Aragocrete" made from the recipes found at www.Garf.org, and it works extremely well. You have complete control over the shapes and consistency by adjusting the mix, and by adding things like small plastic shavings you can encourage rapid encrustation of coralline. The biggest problem is that is has a very long curing time, and if the rock is not fully cured you *do not* want to use it.
My favorite alternative, and the reason for my post, is aragonite rock. I used to be able to get it several years ago from a LFS in Seattle that has since closed. This stuff had fairly good shapes, nice porosity, and grew coralline quite well.
Does anyone know where I can get clean aragonite rock? I plan on setting up a larger system in the next 4-6 months, and would like to have dry rock that I can use to plan the basic structure, without regard to how long it is out of water. I want to make a strong, open structure that will keep everything off of the sand as much as possible, and will drill the pieces and join them with cable ties, epoxy, etc.