Tower of Lies - missed the crystal

BarbaraBinSJC

I missed the crystal at the end of Tower of Lies - ran right past it. Went back to the area after I turned in the quest but the crystal was gone. Am I going to need this crystal to complete the end quest? Is there any way to go back and get the crystal? I would hate to have gone this far in the game to have screwed up the end for myself. Help greatly appreciated.