hear me out here. i think the rooms are all one giant gem, somehow. it has a heart, which is a literal heart, what could be seen as the mouth/salival glands (pearls room), a sort of neck or stumach (amethyst's room). this would explain why there was nothing to see while in that episode where steven was going through an obstical course. you COULD say that the temple was just deleted and replaced with the course while they trained steven, but that wouldnt really make sense. what i think is that they had it far away enough so steven would see only darkness, and NOT a giant gem's rotting body. ALSO, this theory has another supporting argument, in where we have seen that gems can be supported while bubbled in living things, which we saw during 'giant woman', while inside of that bird creature. unfortunately, that also means that all of those gems were lost thanks to Opal destroying said bird creature. ANYWAY... uh... yeah :3 thats what i think