Black Lava Rocks for Fire Pit, 1 Cu Ft, (35 to 40 pounds). Naturally Formed Volcanic Rock Mined in The USA. Varies in Size from 1/2" to 1 1/2" 1 Cubic Foot of Black Lava Rock For Gas Fire Pits. Approximately 35-40 pounds in each bag. Naturally formed and varies in size from 1/2" to 1 1/2". Because its naturally formed, the color may vary from grey to black. Perfect for covering gas fire pit burners and closely imitating a natural coal bed in a campfire. Add an outdoor log set on top of the lava rock for even more realism. We sell the perfect log sets for this on Amazon also! Can also be used as decorative landscape lava rock.