Best material to seal fireplace door gaps for irregular stone fireplace?
the best way to this is with fiberglass gasketing, sometimes called door rope.  its just a fiberglass rope.  you get some hi-temperature sealant for same, inject into the area it is to fill so that it covers stone and the metal assembly.  then, rub it all over the rope (cut to the right length ahead of time) and then simply squeeze into the gap.  different diameters might need to be used, but usually one will do.  tape off all adjacent areas and where gloves as the sealant is quite gooey and makes a big mess.  wipe the area off with acetone after to clean up prior to removing the tape

http://www.mcmaster.com/#high-temperature-seals/=103j65h

http://www.mcmaster.com/#high-temperature-sealants/=103j5zd

they will sell something similar at your local big box building store