Putting volcanic rock inside your grill can give you a better grilling experience and tastier meals. Lava rocks heat up quickly and disperse this heat to the inside of your gas grill. That means food cooks more evenly, with a flavour closer to that from a charcoal grill. Not all grills are manufactured to use lava rocks, but some can be retrofitted for this purpose. If you love grilling with gas, but miss the flavour of charcoal, lava rocks might be the solution for you.
Lava rocks are pieces of crushed volcanic stone placed under the food on a special grate or plate. They collect juices and fat that drip down from the cooking area. When the juices contact the hot rocks, they produce smoke, which wafts up around the food. The result is a flavour closer to that from charcoal grilling than from conventional gas grilling. Using lava rock lets you enjoy the convenience of gas without loss of flavour.
Over time, fat and drippings can build up in the lava rocks, reducing their effectiveness. Built-up fat can also cause what's called a flare-up: a sudden grease fire on the surface of the stone. Flare-ups can lead to burnt food or even a grill fire. Serious Eats recommends covering the grill to starve the fire of oxygen. If flare-ups are a problem, replace your lava rocks with clean ones, or simply turn them over to expose a new surface.
Not all gas grills come with a grate for lava rocks. To add rocks to your grill, install a rock grate about 3 to 4 inches below the cooking grid, and around 1 inch above the gas burners. If the grill has heat plates, you can rest the grate on the same supports. In grills without heat plates, support the grate using bolts through the side of the grill, screwed-in support brackets or just a brick on either side. If you decide to drill into your gas grill to install a rock grate, make sure you won't be voiding a warranty.
Lava rocks aren't the only way to add radiant heat and a smoky flavour to food. Some grills also use metal heat plates or ceramic briquettes. These require less clearance than lava rocks but are more expensive to install and replace. Ceramic briquettes and ceramic-coated heat plates are easier to clean than lava rocks, however, and may need less frequent replacement.