Instructions
Find the button on the north wall of the lower level. This will open a secret room to the south. Pick up two yellow sight cubes.
Place one yellow sight cube in the yellow sight cube socket in the south room.
Take two red blindness cubes from the pile. Activate the lever you revealed in the north room and go down one more level.
Activate the button on the east wall to open a new secret room. Place one of the red blindness cubes in the appropriate socket, then take two purple cubes. Go back to the room with the purple cube socket and place one of the purple cubes there, which will cause the last secret room to open.
Put your remaining yellow, red and purple cubes in the appropriate sockets. Activate the lever, which will take you to the sapphire.