3D Backgrounds???

Just looking for ideas/thoughts on making a nice 3D background for my tank. I have been toying with the idea of cutting pieces of thicker square shale and limestone into pieces about an inch wide and cementing them together to form a natural brick wall look... but the draw back is that not only would it be a pain to cut all that rock but it would take up between and inch and two inches in my tank.

I don't think my skills sculpting foam would look all that good in the tank with all the lace rock that I have.

Any ideas, comments, or experiences making stone/foam backgrounds would be appreciated