This... rocks.

Sorry, it's late, I couldn't resist the easy pun. I came across this super cool idea via Apartment Therapy. Use moss and rocks as a centerpiece. This is super cool looking, not to mention ridiculously cheap. It would look really great in an outdoor spring/summer wedding with an earthy vibe. Maybe you could mix in some branches, different height and shaped white candles or terrariums to mix things up. I'm not so sure how this would work for round tables but I think it's pretty rockin' if you decide to mix it up and go rectangular. This earns extra points for me because I like low centerpieces that you can see over so that you don't have to squint through a kajillion roses to talk to the person opposite you. That, and I'm a fan of anything this easy to DIY, and this freaking cheap!