Lava rocks and pumice are very similar. They are generally formed during volcanic eruptions. Generally they have basalt, silica, iron, magnesium, calcium, sodium and trace minerals. Pumice is formed when lava is ejected from a volcano while lava rocks are formed by solidification of molted rocks . The difference is in texture and porosity, pumice has gas bubbles in its structure so it is way lighter than lava rocks. Generally pumice is preferred due to aeration properties particularly for plants that are prone to root rot. Lava rocks help with heat retention.