Okay, I am assembling what I need to my new BGE. I keep seeing "firebrick" mentioned. Called the local grill store and asked if they had any...they have NO CLUE as to what I'm asking for. So, what IS a firebrick and in the event I can't find one, is there something I can substitute for one. OR...anywhere on line I may purchase one? Thanks in advance to all of you wonderful helpful people!