Grimrock

Head to Ktlak's Domain and defeat the boss. You must pass through the Eternal Catacombs, Olde Scorched Valley, and Nomad's Demise North & South to reach that stage. This isn't imperative right away as you can still collect other essences and items before you head there. Once you have defeated Ktalak, then you are able to reach that Power Gem. I strongly suggest finding the Water Essences first.

To reach the other Air Essence (in Nomad's Demise - North) you must defeat the Air Elementals, and use the second rapier to place on the beach statue. The statue is right next to the blue-eyed statue near the pool under the player's feet. There are two rapiers in the game, one is buried in the Turtle Farm in the Main Beach, the other is in a chest that says "...Thievery will be dealt with accordingly".

The Tomb Key is for later on in the game. There are actually three Tomb Keys, but you will find their use later. The four Power Gems are located in the Main Beach, Greater Grimwern Woodlands, Grimwern Woodlands, and the Southern Zhaoze. Acquiring two is enough to attain the Water Essence, so try exploring other areas. Don't Fuse the four Power Gems until you have acquired the Water Essence from Greater Grimwern Woodlands. The Brass Key is used in Grimwern Woodlands right near the entrance. You need to use it on the wall where the Undead rose, then return to the entrance. Afterwards, you may use the Cube Key to enter the shut-off area (where the spear door is).

Just pull both levers to enter the Museum area, leave the others that are located elsewhere. After, walk around the back of the museum to find the hidden lever.