Okay -- so I'm trying to do the quest in which I need the Resonance Gems, but unfortunately, Revus Saravani died, and I only have three gems now.
The wiki page on Fahlbtharz says that the Resonance Gem respawns -- and yet I've seen no evidence of it ever respawning again. Also, I cannot get back into Nchardak at all to get the other one. Can anyone offer help, please?
For Fahlbtharz to respawn you'd probably have to stay away for over thirty in-game days straight (maybe ten but thirty just to be sure), i've never tried it myself so can't say for certain the gem does respawn.
There is a section about getting back into Nchardak aqueduct on the wiki page here, if the outside door is sealed apparently fast traveling to Raven Rock and back may let you get in (again no idea if thats true though).