The Small Black Lava Rock from American Fire Glass offers a high quality fire media for your gas fire feature. The naturally occuring lava stones are produced when volcanic lava reacts with cool ocean water and hardens to form a temperature resistant rock that makes a perfect enhancement for a gas fire pit or fireplace. The stones keep their naturally jagged shape for a classic appearance and the 20 pound bag includes rocks that range from 1/4-inch to 1/2-inch sizes to enhance your fire feature.