I would choose peridot (I've always known it as that, but it's called Olivine too). It's not the flashiest of gems, but it has a simple beauty and I think it fits very well with my writing.
It's a symbol of protection, which is a major theme in my book. A group of eight teens is sent to survive on a new planet (it's fantasy/sci-fi/adventure) and there is a very strong interdependence about them. They may not always get along, in fact they rarely do, but the would do just about anything for one another. They've been together through thick and thin and there is a major protection element there.
The color was another main reason I chose peridot. It has that clear, earthy green color. My book has a lot of fantasy and sci-fi elements, and the only reason the group was sent to the planet in the first place was because of the polluted and overpopulated state of the Earth. (There's some other reasons, but I'll just stick to that. It's easier to explain, haha!) The peridot stone is that lovely green, something that people from Earth in this time, 2254, don't see hardly at all. It symbolizes all that they hope to see in the new planet. The new planet is very beautiful, but harsh.
I didn't know this, but I just googled peridot and found that the ancients believed that if you dreamt of it, dangerous times were ahead. That's a coincidence that really fits with my book because it's dangerous situation after dangerous situation. They're constantly in trouble and there are quite a few life-or-death moments. Their goal is to simply survive.
Apparently it is associated with luck and courage as well, which is a big part of the story. Sometimes they got out of sticky situations just by pure luck. But all the members of the group possess a lot of courage and bravery, and a few fight to the death.
One meaning has it symbolize underlying love and admiration, which fits with the romance story that is incorporated into the action and adventure. My characters Sarah and Tyler are polar opposites and most of the time they aggravate each other to no end, but in the end there really is an underlying respect, understanding, and attraction.
For the 'too long, didn't read tl;dr' crowd: "I can has choosed peridot as story gem!" :P
My second choice would be opal. I think it's one of the most beautiful, but under appreciated gems. It has so many tones and shines, it's absolutely stunning! It's my mothers birth gem, so I got her a ring with that as the stone. I didn't know that it's bad luck to get an opal for yourself! That's an interesting fact.
(Yeah... sorry I wrote my answer as a novel in itself.)
My previous knowledge, along with google and a websites that listed some information.