Using Lavarocks or Expanded Clay to Grow Marijuana. Gravel, sand, perlite, rockwool, oasis cubes, coco, expanded clay ('poprocks' or Hydroton/Groton/ L.E.C.A. and other names), and lava rocks are common choices for Hydroponic mediums. Lava rock and poprocks are popular as they are inert, do not absorb water, provide good aeration, are …Web