Okay.. I have not seen any good advice yet and I'm about as romantic as a lion. (find it, screw it)
GET HER EARINGS WITH HER BIRTH STONE!!! Then, next year, if you can't find anything else.. you can add to it and get her a matching bracelet with her birth stone. Then, after that, you can get her a matching necklace with her birth stone. (I did it and my girlfriend cried every time. Then I broke up with her.)
A ring is a big NO NO!
Birth stone shows that you are thoughtful.
Also, DO NOT GET HER PLATINUM JUNK. Real jewlery is made out of gold. Certain circuits and microchips are made from platinum. Platinum is a fad and just a way to be expensive. It has no real luster and all the platinum you see at a jewlery store has been shined up like hell. Get her a platinum ring and it will be dull (like pewter) in a few months.
EDIT: BTW, pretty much EVERYTHING with her birth stone will cost between $50-$250.. The earings will be the cheapest - but they can usually be worn the most. My exgirlfriend wore the earings I got her for a year straight (until she started working out).
Also, when you buy them, tell them you'll pay ~75%. (But don't say you'll only pay "75%". Tell them you'll pay $75 if the cost is $100. They'll usually take it. They will always give you 10% off if you ask (in malls, where they charge a lot).
Also, try a place like Sams Wholesale, Service Merchandise... They have nice gold, diamonds, birth stones, etc and they are usually 30% or so less than mall jewlery stores. Remember, it's not where you got it from that matters - it's what it is that you got. You owe it to her to get the nicest and best crap for the money you have.
Also, jewlery is crap - but it lasts forever. So you'll probby get it thrown back in your face when you break up. (sell on Ebay, make $$$)