Lava rock is a common material for heat distribution in gas grills. Above the gas burners sits a rock plate. The lava rock is spread across the rock plate, and absorbs the heat from the burners, distributing the heat evenly across the grill. While lava rock is the least expensive radiant heat distribution method, you do need to change it once every season or so. It can also absorb grease, due to its porous surface, which can cause dangerous flare-ups. These disadvantages lead some grill owners to seek other options.
Pumice stone is another type of volcanic rock that you can use for heat distribution. It is a little more expensive than standard lava rock. The surface of the stone is less porous, which reduces the risk of grease fires and flare-ups. The heat distribution capabilities of the stone are very similar to that of lava rock.
A very clean heat distribution option is ceramic briquettes. You align these across the surface of the rock plate in a similar manner to lava rock. The chief advantages of ceramic briquettes are cleanliness and durability. Grease and debris that hits the surface of a briquette immediately cooks away. Any remaining residue can be burnt off safely by turning the briquettes upside down. You can turn briquettes over periodically to extend their usefulness. Unlike lava rock and pumice, they are unlikely to require frequent replacement.
Heat Distribution Plates
Heat distribution plates are metal bars, sometimes coated with ceramic material, which you lay above the burner to redistribute heat. They tend to burn a little hotter than the stone variants, because they provide a lesser degree of insulation. Due to the heat, grease usually vaporises as it strikes the plate, which prevents the build-up that causes fires with lava rock. The sturdy cast-iron varieties also almost never need to be replaced.
Infrared Radiant Burner
If a new grill is in order, an infrared radiant burner might be just the ticket. Rather than relying on a separate mechanism to distribute the heat, the radiant burner is placed at the back of the grill to create an even heat across the surface. Radiant burners are designed for use with a rotisserie, so another option might still be necessary for times when you don't want to use it.