Stones, while not a must-have, are often placed at the bottom of fire pits. Lava rocks are a popular alternative to stones, especially for a decorative touch for fire pit tables. The purpose of stones at the bottom of a fire pit is to provide proper drainage. You can also substitute stones with sand, gravel, or even dirt. Some people also use filler, especially concrete, though this tends to be more expensive. Ultimately, it’s a matter of preference for what you want to fill the bottom of your fire pit with.