Hello again!
You'll need the fourth green gem to go further. In the dark room, there seems to be obstacles where there shouldn't, and some obstacles aren't what they look like... Try to go through and if you can't, try to throw something.
Solution
in the south part, you can spot an alcove with a plate on the floor, behind a couple of forcefields. You can't go through those but you can throw an item to the plate. It will deactivate a forcefield behind you so you can pull the lever. At this point you can go through the forcefields and reach the alcove.
When you have the four green gems, you still have to figure out in which order to put them...
pay attention to the order of the four schools of magic...
You will need the egg for a secret in this level (not capital if you don't find it). You should read carefully one of the notes in the Library regarding dragons.
By the way, good work in the Old Chapel, I never thought it would possible to achieve that!!
