ToME: the Tales of Maj'Eyal

Other people have obviously managed to get resonating diamonds without killing the source: Zemekkys must have used one to make the original slab for the portal tests and if it hadn't been consumed in the process he wouldn't need a new one. The orcs must have had at least three: two for making their portals and a spare for you to loot to make your return portal.

Why do you have to kill Briagh and turn a renewable resource nonrenewable?

Are you suggesting that perhaps we should tweak that fight as a non-kill battle? I like that question.

Perhaps you fight him and then at some point, the stone drops and he flees…

Maybe it could be made like the Grand Corruptor fight, where once he's low health he communicates with you somehow and you can choose to spare him or kill him. Adding choices like that always make things more interesting.