Aquamarine helps me.. I have some runes made out of aquamarine and I find it helps keep me focused, calm, and emotionally secure.. I don't 'freak out' as much when I am carrying them with me.. Clear Quartz helps me stay focused and harmonious too.. lunakasha is right though, you should find some that work for you.
I didn't realize when I bought my runes they would make me feel so calm, clear headed, and focused... it wasn't the reason I bought them.. but the instant I held them.. I knew! Should work the same for you.. do you have a place you can see a variety of them in person? Holding a few of the same stone will help you decide which properties work best for you.. if nothing else, just take a peek online at some sites that show gems that calm nerves, relieve stress, and keep you centered.. then buy them based upon your reaction to their colors and properties.. thats about all I can suggest unless you can see them in person
Hope that helps a little.