In regards to the bag with the power gem, there is another way to reach it. If you climb up so that you're on equal elevation with the bag, then side-step off the cliff so that you're facing the bag, there's a split-second moment before you fall where you can click and grab the bag. I did the same thing with the Earth Essence on the pedestal on the small island surrounded by water, long before I even discovered how to get into the Excavation Site and complete the puzzles there to reach the island "proper".
As for the rapier puzzle - there are two rapiers in the game, and you'll need both of them to solve puzzles. So hang on to one until later.
But now I'm curious. In how many places am I supposed to use a Power Gem to open something? The reason I'm asking is
I've already found all four power gems and merged them into the Balance Essence. Right now I'm only missing a Fire Essence and a Water Essence to be able to reach the second Balance Essence and therefore "solve" the big puzzle on the Elemental Plains. If the Power gems are needed to reach either one of the two aforementioned Essences, I'm screwed.
On that note, the three coloured gems.
Are they needed to get any Essences? I found the red gem, placed it in a niche in the Excavation Site and something happened (though I never found out what exactly) and I found the blue gem, but I never found a place to put it like the red gem. Then I found the well and I dropped both gems into it, so I may have screwed myself over with the blue gem.
Meanwhile, can anyone give me hints for the following troublespots I'm in?
Nomad's Pass - Southern Gaol:
How do I free Raisa? The lock requires a Gold Key, yet the clue suggests "a thief's weapon of choice".
Halls of Ktalak:
After solving some pretty neat puzzles, I've reached a square where there's a teleporter behind me that takes me back to the start, a button on the north wall that doesn't seem to do anything, and a pit to the east that keeps opening and closing, with a lever on the east wall. Pulling that lever doesn't seem to do anything.