The best replacement for lava rock is mulch.

The best alternatives to lava rock are rated as following:
★★★★★  mulch
★★★  river rock
★★★  rubber mulch
★★  radiant charbroiler
★★  ceramic briquettes
★★  fire glass
★  perlite
★  hydroton
★  pumice
★  bio balls
★  flavouriser bars
★  heat plate
★  flame tamer
  meteorite
  brick chips
  penroc
  radiant
  ceramic brickettes
  briquettes

Your alternative for lava rock is not listed? Suggest!
Speaking about lava rock, one probably shouldn't forget about gas grill.

Other search terms related to lava rock and their ratings are:
★★★★★ gas grill
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Google Trends

Scale is based on the average worldwide traffic of lava rock in all years. Click on graph for more.
lava rock
mulch
river rock