Grimrock

I finally managed to find the shrine of balance following the clues given by several statue heads. The thing is, I've only got one Power Gem left and the only unexplored region is Castle Nex. I am going to list rough locations of the gems I found in the next spoiler.

4x Twigroot Forest: I didn't keep close tabs at the start, but I found all of them before I left Twigroot forest
3x Keelbreach Bog: one inside the Herder's Den, one by solving the 'Reflexus' riddle, one on an island in the NW with four fallen statue heads
2x Ruins of Desarune: one inside the final room before the Archives, one inside the Lexicanum
2x Hamlet of Stormbreach: one inside the House of Needles, one in the Sewer Reservoir
2x Crystal Mines: one in the Abyss level, one in the Entrance level
3x Cemetery: one above ground to the north, one in a zombie pit inside the Catacombs, one after slaying Orul in his crypt
1x Pyramids: in the Tomb of Rites, before going up to the top floor

Then I accidentally figured out that you don't actually need the fifth element to enter the castle. Once you have placed all of the four elements on the pedestals the castle gate will open when you walk towards it. I'm currently stuck in a "test chamber" in the castle and haven't found the gem yet, but I'm guessing I'll find the last one somewhere in there.

So you are missing only one gem? Can you remember how many you got from each of the major zones?

I think the ones not listed on your list is the one you get after killing a magma golem in the desert and one who was in the "Forgotten River" area.

Yeah, except even after you get through castle Nex, you will eventually need to find all of the power gems. I am unfortunately short 1, and haven't a clue where to find it. It doesn't look like I am missing any of the listed ones, but there are a few tiny spots in Nex I haven't explored...if its not there I'm in trouble.

I eventually found the one I was looking for past the Power Me Up thing in the crystal caves.

You get one power gem in Castle Nex itself, after killing the dragon. Did you get that one?