I use regular river pebbles (pea gravel). Found it at a nursery. Very heavy though. I don't see why lava wouldn't work. Except it will get clogged up and will retain a lot of junk in the pores. On the flip side, it will also allow a lot of surface for beneficial bacteria to grow on.
You may want to read this thread too:
http://www.orchidboard.com/community...highlight=lava