You move around the room testing the floor and finding where a solid path lies. As you walk the glow on the brass box follows, its beam of light shining directly to the jewel in the center and then being cast directly north into the far chamber.
You identify the following path in the map. The white area is the real tiled ground, while the shaded areas are the illusions leading to the acid pit about 15 feet below.
You now stand at the north side of the room. The beam of light shines directly north into that room, but you can see nothing in the room. It is as though a wall of darkness blocks you from seeing past.
The other problem you see is that there does not seem to be a solid path to the jewel in the center of the room. All you have found so far is the illusionary floor as you circle it clockwise.
The ticking continues, and even as you watch, the stone door drops back into place, locking you inside while the gnomes wait outside.