Lava is molten rock. When it is very hot, it glows ‘red’ to our eyes. As the surface of the lava cools (faster than the inside) it will look darker. Some cooled lava is black, especially basaltic lava, which is most common in places like Hawaii.

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Of lava rock, glass and stones, each absorbs and retains heat quite differently. Lava rock is very porous and hardly holds heat at all. Stones are at the other end of the spectrum. They can hold and radiate heat long after the fire is extinguished.

Instead, use lava rocks for your fire pit or lava glass beads as a filler for your fire pit. They are a safe way to create drainage and make your fire pit look nice. Sandstone, river rocks, natural rocks, and gravel are not ideal fill for fire pits because they are more likely to crack or explode under high heat.

Sky blue

Lava rocks are very porous like a sponge. Water can get trapped inside of lava rocks when they get wet. When the wet lava rocks get heated up by the fire, the water inside turns to steam and expands. This creates pressure inside of the lava rocks which can cause them to explode open.

Are lava rocks toxic? Note: Lava rock is only suitable once it has been weathered and cleaned. When fresh, it is quite poisonous. Be sure to obtain lava rock from an aquatic dealer.

Round Fire Pit Calculator We recommend 2″ – 4″ of lava depth for most fireplaces. For rectangular and square fireplaces, use the same front width as back width. All measurements should be made in inches.