Long ago gas grills used stone to separate the flame from the food. Typically, found in lava rocks in the lower priced grills, this barrier absorbed heat, evening the flame that caused drippings to quickly combust producing smoke that flavored the food. These days, grease is managed by channeling it away from the burners to reduce flare-ups. This action decreases smoke, though some might complain, it eliminates authentic grilled flavor.
Find out how to Convert a Gas Grill to Lava Rocks.