Lava rock is often used as an inorganic mulching material. As an inorganic material, lava rock does not add nutrients to the garden, but it does last a long time.
While lava rock does not add any nutrients to plants, it does absorb heat very well and releases it at night while also acting as an effective weed suppressant if it's coupled with a weed barrier. Red lava rocks add considerable color to gardens, and its texture provides a visual impact all by itself. Red lava rock is often used in desert landscapes since it blends in with the surrounding environment, but more neutral black lava rocks are also available. Lava rock is also very lightweight, which leads to lower costs compared to other mulches, and it makes it much easier to transport.
Despite its lightweight, lava rock is a difficult and costly mulch to remove. Lava rock also easily blows or washes away when it is left in an exposed location, so it is best to install the lava rock at least 1 inch below sidewalk level. When lava rocks do blow away and end up in a grass lawn, they are at risk of damaging law mower blades due to their sharp edges.