I would use rapid rooter or jiffy pots from your local lowes or home depot or kmart or walmart and then place that into your lava rocks with a clear plastic cup over it until you get some head. Lol. Lol. Lol. Seriously.
CG on lava rock. It is actually the most prefered medium for roots if not the safest as any jogging with DWC in hand will surley slice the roots with those sharp edges. I read a study that was legit and schoolish where lava was the big winner.