I think Man-of-the-Wold's ideas here are very compelling. Wish I'd found them three months sooner so I could plagiarise it for a term paper.
In seriousness, though, I think you're really onto something there. It rather made me think of the process by which diamonds are made - pieces of coal, left underground and under pressure for long enough, turn into precious gemstones. It's something to think about. You could probably make an argument for the significance of the weight of the earth overtop of them as representative of the weight of the entire Quest, which changes the characters and, in most cases, elevates them. Think about it:
[*]The obvious one - Gandalf, fighting the Balrog in Moria, gets upgraded from Gray to White.[*]As was mentioned before, Sam discovers his heroism after passing through Shelob's lair.[*]Aragorn fully claims his status as king.[*]Merry and Pippin find ways to contribute.[*]Even though Frodo ends up emotionally destroyed, he is elevated in the sense that he is granted permission to go into the West with the elves. [/list]
In all of these cases (and I'm sure there are many others, but I don't have the book in front of me), members of the Fellowship who have passed through some of the various caves in the story become something better than what they were - in essence, they start as coal, and turn into diamonds. (Gandalf, again, provides the best example, starting off gray like coal dust - sort of - and ending up white like diamonds.) It isn't necessarily true that all these characters' changes come directly from experiences within the caves, though; what does anybody make of that?
Also, what about characters who don't go through caves? As far as I can remember, Éowyn never goes through a cave, but she undoubtedly changes and develops throughout the text. (That's actually what I ended up writing my essay on.) What contributes to her change from coal to diamonds, so to speak?
If I come up with anything later, I'll come back. In the meantime, I'm excited to hear what you have to say.