I am so sorry not to have replied earlier. I do try to keep an eye on the forums but I completely missed your post. It's old age
I'm sure you've cracked it by now but the red gems give you health and the yellow gems give you adrenaline. Also, I'm glad our Guide has helped you (from your post elsewhere). Astonishingly, we're still getting 150-200 visits a week.
If you've enjoyed NVN, do give Lost Company a try. Most enjoyable
Greybeard/AncientMariner