Partly suggestion and partly bug, but suggestion is the bigger part so I put it here.

The reward on this quest is really low currently. Fire Agates are useless, selling for maybe 3 gold each if you're lucky. You rarely find more than one diamond, and its always been small in my experience.

I'd suggest toning this quests reward up just a little, even with a fire resistant priest *Endurance spells* I wound up using 500 gold in potions just to finish the quest--only to find nothing but fire agates today, or toning down the flaming rock damage. 15-30 damage to get a 3 gp rock is disheartening.

AND

The priest won't give a reward for diamonds at all, he just takes those.