I am guessing all the other gems could be sold safely but nobody got far enough to let us know.
The red gem you get from the skeleton can be sold, but the journal says you need it for a quest. It didn't fail the quest when I accidentally sold it, but I think I'll keep it. I think you get at least one emerald that's worth around 600 gold.
For me, the game might as well end as soon as you get to the sewer, though, since I can't handle the mimics nor the goblins. Got any pro-tips for that? Maybe some way to get around 7k party XP and a boatload of gold before getting there?
After getting past the group of centipedes you can convince the first group of monsters (kobolds?) to not attack with multiple skill checks. Then you should go South and there is a Grey Goblin who’s guarding a Portcullis. Talk to him and tell him you will fight the Green Goblins and you’ll get extra XP and rewards from the Goblin fight.
Next, talk to the flying thing by the pit and have it transport you across the pit. (You can actually jump across the pit with a Fighter, but I think it’s bugged because the Goblin dialogue doesn’t trigger for me when I jump.) Once you are across the pit, the Goblins should each try to convince you to join them. Pick whichever group, but you get extra XP if you talked to the Gate Guard beforehand and you join the Grey Goblins. And then the battle begins.
Perhaps I was just lucky, but they raped my party at first, and I had two characters left, each with 1 HP and I just moved them around the map while the goblins attacked each other. In my case, the Grey Goblins absolutely destroyed the Green ones. However, there were only 7 goblins left alive at the end of it, which meant my XP reward was lower than it could have been. Didn’t matter to me because that fight was long and I was not doing it again.