Rocks

Andy told me once that all of geology is the study of pressure. And time, of course. (467-468)

Not so subtle here, Mr. King. The rocks that Andy finds in the prison yard turn into a symbol of how he goes about things: slowly and patiently, but with incredible results. The rocks don't look like much when he finds them, but with a little elbow grease and a lot of time, they can turn into something marvelous.

For example, take the polished stones Andy gives to Red:

I didn't even dare touch them, they were so pretty. There's a crying shortage of pretty things in the slam, and the real pity of it is that a lot of men don't even seem to miss them. (135)

In an awful place like Shawshank, beautiful things don't attract much attention. No one notices them in the yard, and even if they do, no one thinks they're especially beautiful. Only Andy does, and by polishing them up, he can show their beauty to people who need it. The rocks help Red realize that wonderful things do exist in Shawshank, if you know where to look.
The tragedy of Shawshank is that no one else comes to the same realization as Red. The hope is that someone who does—like Andy—can help teach others to find beauty in places they wouldn't normally look. Spreading the love, one polished piece of quartz at a time.

Setting:Shawshank Prison

With the exception of the end of the book, most of this story is set at Shawshank, a fictitious prison located in Stephen King's home state of Maine. King doesn't spend much time describing it in the book. We get the word "grey" a lot, that's for sure, as in "these goddamned grey walls" (194) and "big grey clouds scudding across the sky above the grey walls" (247). King gets to the point and probably figures we can imagine what a prison might look like, so we get a lot of "grey" and a few other tidbits like how horrible the guards are and what happens when the sisters get to you. It's basically an awful place and when you go there, you lose everything.

They give you life, and that's what they take – all of it that counts, anyway. (33)

Shawshank's awfulness kind of seeps into you over time. It's not like you walk in and see prisoners getting brutalized in the yard, or line up for your weekly assault or anything like that. It gets to you over the years. Slowly, inch by inch, you lose who you were and become an extension of the prison. You can't escape, even if they let you out.

When you take away a man's freedom and teach him to live in a cell, he seems to lose his ability to think in dimensions. (386)

Shawshank is horrible, but its horribleness reveals itself so gradually that you don't even notice.

For several decades, Andy and his unbreakable sunniness go head to head against Shawshank's creeping dreadfulness. Shawshank almost becomes a character itself—Andy's gotta beat it if he wants to survive. The wardens come and ago, the evil prisoners get shipped off to different penal institutions, but Shawshank? It's always there, waiting for him.

Luckily, it's got cruddy walls.

The walls of Cellblock 5 were solid enough, but they weren't exactly dry and toasty. As a matter of fact, they were and are pretty damned dank. After a long wet spell they would sweat and sometimes even drip. Cracks had a way of appearing, some an inch deep, and were routinely mortared over. (466)

If Shawshank's a character, then that's its Achilles heel. It just takes some patience to get to it, and as we know, Andy has got lots of that.

In contrast, Mexico serves as a striking juxtaposition to Shawshank. We never see it, but we know it's wide open, forgiving, and doesn't force you to share in its bleakness.

'You know what the Mexicans say about the Pacific?' I told him I didn't. 'They say it has no memory. And that's where I want to finish out my life, Red. In a warm place that has no memory.' (328-330)

Mexico helps give Andy strength to fight Shawshank. Even if it turns out to be some cruddy little beach full of windblown shacks, it's the idea of Mexico that he needs. His imagination makes it out to be beautiful, and regardless of whether it really is like he dreams, the hope of one day being free helps Andy stave off Shawshank's grey walls.