Lava rocks are often used for gas barbeque grills and can become quite dirty with food and grease droppings. You cannot use a wire brush or wash them in water as you might ceramic briquettes found in other gas grills. Instead, you must burn them clean. If however, the lava rocks are breaking apart, it's time to replace them rather than attempting to clean them.
Turn the lava rocks carefully over so that the greasy side is facing down. Do this only when the grill is completely cool.
Lay some aluminum foil over the grill grate. This will help trap the heat in for a more efficient cleaning.
Turn the grill on high for up to thirty minutes. Your grill will start to smoke due to the burning grease. After it stops burning or the half hour is up, turn off your grill. Excessive heat for a long period of time can warp the grill.
Allow your grill to cool completely. Turn it back on high again to repeat the cleaning if necessary. Turn off after thirty minutes or when the smoke stops, which it should by that amount of time.
Allow the grill to cool and remove the foil. Your grill is all ready to use for cooking with clean lava rocks.