The layout of the cave was changed in Update 3 (September), so any video from before then will not be useful. From what I remember though, the gem is on the upper level of the only two-level part of the cave, which should be right behind the boss. (The boss is on the upper level, right?)
See Map, the Anguish Gem is marked with the white quest marker.
In case anyone is still looking for this answer, in the new layout, when you get to the T junction in the lower right center of the map, if you head right you'll go down to where the boss and path back up to the entrance is. If you head left there, you'll come to a catwalk area where the gem is.