What is Volkanix Terra Lava Rock? Lava rock is a form of igneous rock called Scoria that is created by the eruption of volcanoes. Scoria is formed when the molten lava solidifies, in that process air is trapped creating the porous, brownish red lava rock. The Volkanix Terra range is produced and graded specifically for the horticultural market and each rock is sized between 8-32mm. It has amazing heat and water retention properties and is also resistant to insects, fungi and disease. Our lava rock is sterile, inert, PH neutral and organic. How to use Volkanix Terra Lava Rock? Volkanix lava rock can be used a top dressing or mixed into your existing soil.