I found the Crytal that's "in the wall" before getting the quest and now the crystal vanished... I don't have it and it's not hiding either.

Can someone give me the name of the crystal für the get-cheat maybe? Pls. I wouldn't want to replay the whole quest!

Restart the level, and remember to read the plaque on the floor near the bottom of the statue. When you opened that plaque ONCE before getting the crystals you will never have to do it again and none crystal will ever disappear. Even when you later might replay the whole game (i did it 50+) no need to read any plaque anymore.