None of our grills feature lava rocks/ceramic briquettes, and when we made our first line of Genesis gas grills back in 1985 we specifically designed them to NOT feature lava rocks/ceramic briquettes.
Using lava rocks/ceramic briquettes in a Weber gas grill will void the warranty on the unit, and could present safety issues.
Instead of lava rocks/ceramic briquettes, we came up with what we call Flavorizer Bars, shown below.
Flavorizer bars sit over the burners, and are designed to perform the same sort of function as lava rocks/ceramic briquettes, but much more effectively.
The bars heat up from the burners beneath them, and any dripping grease and seasonings from the food that hit them are vaporized, and the vapors circulate through the grill, imparting more flavor to the food being cooked. They also protect the burners from the drippings to keep them free of clogs and to reduce flare ups.
Lava rocks/ceramic briquettes are prone to flare ups, and they are incredibly hard to clean. Flavorizer bars can be easily brushed or scraped off in just a couple of minutes.