I used a cloud texture with a color ramp, set the dark values to black and light values to transparent. After that, I upped the texture size to get large contiguous patches of both values. I set the black value’s influence to normal to make a bump map (blender internal).
If I want, I can duplicate that texture and set its influence to specularity, so the black spots are not only raised slightly but shiny too.
What i’d like to do now is give the raised portions some surface bumps, like a grainy/sandy look. Does anyone know how?