According to this thread, when the labels look like this the gems are in the Blue Box:
Blue: The Gems Are In The Only True Box
White: Only One Statement Is True
Black: The Blue Box contains the Gems
There is a near-identical setup where the Blue & White boxes say the same thing, but the Black Box says: The Gems Are In The Blue Box, which is the same thing, worded differently, but the Blue Box is empty. This pretty much chucks any fairness out the window.
There is an 8th location past the outside entrance located near the blue flame elevator. After breaking the wall described above and heading inside the entrance, you will find a second breakable wall that leads into the area with all the carts that had to be rearranged to press a button.
