Dragon takes groups Wizard, told to kill his rival and take his gem-heart

*Group is coming back and heavily damaged*
Dragon: Good, now just set the gem down and I’ll get your friend
Sorcerer: Okay *sets down the gem*
(Me) Fighter: No, we want our friend fi-
DM/Dragon: The dragon rears back and prepares a giant fire breath at you all, you each get a quick reaction
Sorcerer, Bard, Artificer: Run and dodge!
Ranger: I fire an arrow into its mouth
Fighter: I run for the gem and head for a lava pit
Group: WHAT!?
DM/Dragon: Hah, alright, the dragon blows out a ball of fire, everyone make a Dex save, dodgers with advantage.
*Everyone rolls, only the Artificer and Fighter fail*
DM: So that’s the damage, weirdly the Fighter took absolutely none
Fighter: I look right at the dragon and hold the gem over the lava. “Give us our friend, or it goes in the lava!”
Dragon: Fool! I am fire! I will find it! GIVE ME THE GEM LITTLE DWARF
Fighter: “Fine, spend the next century looking for it” And I jump into the lava with the gem
Wizard: *not with group, but in the room* BAHAHAHA
Ranger: NO!
Bard: What?!
Sorcerer and Artificer: Dude!
DM/Dragon: Really? I’ll give you a second chan-
Fighter: No, that dragon can spend his life looking for my chard corpse holding onto the gem while my team runs
DM:…Ok, well that’s one way to find out the gem gives you immunity to fire/heat/lava/all things that could possibly burn you.
Fighter: Really!?
DM: So ya, you all see your Fighter basically say the most badass one liner to an angry dragon before, seemingly, heroically sacrificing themself in an attempt to give you all a chance to run. And before you ask, no, none of you would even be able to guess the gem is keeping them alive. Ya’s taking this chance to run?
Group: *all in disarray*