The problem with using a product like lava rock, as mulch is it is difficult to work around.
Rarely can any plant be left alone and not eventually require attention and when this happens if you have to deal with lava rock, you may regret using that material. It also creates extra heat around plants and often does not look natural like tree bark, shredded leaves, hulls and other mulches that will break down. These products add nutrients to the soil when they breakdown and lava rock does not.