Need Adventure Idea

Not sure if I have the right board but I'll ask anyway....

I am running a quick AD&D 3e adventure tomorrow evening. I plan to drop the characters into Mirkwood. They are being chased by Goblins through the woods and find a cave that looks suitable to hide in. A dying elf is in that cave, having barely survived his venutre into the dungeon below the cave. I have a quick dungeon set for them where they retrieve a red gem shaped like a fist. I need an idea of what and why that gem exists and why the elf may have wanted it. My Middle Earth knowledge is limited to the Hobbit (for now - I am reading Tolkien daily now to learn more). So I would appreciate any help you might provide.

Thanks in advance.