This is one of the toughest puzzles in the entire game. Well, the room that the emerald is in is called the "Plover Room." Often in the game a word will be used to go to a room nearby if you don't feel like remembering the exact direction to go. (You can type slab when in Bedquilt to get to the Slab Room, for example.)

So, typing plover from a room adjacent to the Plover Room will get you to the Plover Room. But if you get goofed up and type that in the Plover Room itself, you will discover that "plover" is Yet Another Magic Word that teleports you to the "Y2" rock! And typing it again teleports you from the "Y2" rock to the Plover Room.

So, knowing this, it is a simple matter to go to the "Y2" rock, lamp in hand, and type plover to go to the Plover Room, bypassing the tight squeeze that forces you to drop the lamp. And then you can go ne and visit the dark room. It's only one room, containing one treasure (and a rather whimsical message).