Actually guys, with this post I hope to stop the discussion once and for all because those who don't know what to do are only being confused by posters who have done it but have no clear details on what to do with the Invoker Jewels.
In the final Interior Halls where you first meet and combat Stone (before he runs to the Arena) there is another hallway other than the short one to Arena teleporter. Down this other hallway is another small squad of Black Ring and also the re-spawning Summoned Armors. The trick of this location is that the machine on the other side of the room has to be overloaded. The problem is that it will require 5 gems to do it.
Now,notice the green gems on pedestals right nearby? There are 4 of them, so simply target them with your reticle (the "targeting circle" ) and take them as you would any other item. But the trick of it is that theres only 4.....you need 5 to overload the machine. Can you guess where the other Invoker Gem is? Indeed, it is the very one mentioned by several of you as being discovered after the Arena conflict with Stone.
Oh and btw, for those of you who may find Stone a challenge, use of Explosive Arrows skill makes him and most other bosses easy as pie, since at range they tend to favor lower damage/easily avoided attacks and the Explosive Arrows skill is quite effective at whittling them down.
Once you have all 5 of the Invoker Gems go to the control panel and it will ask if you want to use the 5 gems and simply affirm it and the machine will shutdown. As well, a small chest will fall at your feet with a mid-level magical item in it (I got some crappy necklace...didn't bother to reload to see if its a random-cycling item or static.)
And finally, as for the 4 massive steel doors at the end of the tunnels in the Arena, I believe them to be purely for show. Consider that you must eliminate the 4 Chief Minions and these would seem to be a theatrioal approach to appear as "holding cells" for them as any good arena has to have for its gladiators. Unfortunately no kewl CGI movie of seeing these creatures emerge, they simply pop into Arena in a flash of Teleportational Magick.....Weak! Anyhoo, i wasn't really concerned with seeing if I could get into Mumbo, Jumbo, Gumbo and Dumbo's little lairs since there doors are unpickable at 5th level and no keys drop from them or Stone.
*oh and also, for those of you who haven't actually fought Stone in the Arena yet those of course are not their real names, but since they weren't very impressive combatants I chose not to use their unduly impressive-ish names since all they are are a slightly overpowered Troll, Beholder, Demon and a new incarnation of a creature some sorta "hovering tentacle Cyclops" thing.