Before a Quartz Door                                   -      N      - 
(-------------------------------------------------)     W <---(M)---> E
                                                        -      -      -

The hallway here is still made out of quartz and must of taken a very
long time to chisel to such perfection as you see in this part of the
tunnel.  To the north is a giant door made purely of the hard translucent
rock.  The door must weigh well over a ton and only someone with titanic
strength could possibly hope to open it.  You don't spot any sort of locking
mechanism on the portal.