Me and my sister have found every obsidian chest except the one in the labyrinth. What doesn't help is that we're not even sure we've been in the labyrinth before and have no idea where it is. Can anyone give us any hints?
Since you're doing the obsidian quest, you would have had to have done the Labyrinth ... it is the dungeon in the Necropolis were you had to fight the dean in order to get the student badge. The obsidian chest is in his chamber.
You have to do some of the Labyrinth instance again. You need to defeat the Silver Sentinel, then open the bridge portals and defeat the three wandering bosses. Then you open the North Bridge portal and go inside. The chest is there. When you retrieve the contents of the chest, you can leave.