it is quite common to find orchids grown in lava rock (large pore pumace) especially when they are imported from hawaii. we actually get our lava rock from lowes it's sold for landscaping (and gas grills) in 50 lb sacks. we use it mainly for dendro's and vandas as they prefer a really fast draining medium.
btw, there's an orchid thread over at indoor gardening and greenhouses. lots of great pics come check it out....
cheers,
doc
ps an after thought, orchids in lava rock like a little extra fertilizer. the rock doesn't give nutrients back like a bark mix does.