American Fire Glass LAVA-M-10 1/2" - 1" Medium Black Lava Rock 10 lbs. Any amazing gas fire feature begins with a proper and solid base. You can use fire glass or fire stones in your fireplace alone, but for deep set fire pits it is best to lay down a lava rock base. Not only is it lighter weight by mass (ten-pound bag of lava rock will fill as much area as twenty pounds of fire glass) but is more cost effective for your fire pit project. This material can be utilized inside and out and can withstand high temperatures in any fire feature because of the natural process magma goes through when cooling. As the magma nears the surface and cools, the rock releases gas bubbles that forms the coarse rough surface in the lava rock.