I would steer away from a ring on a necklace, not because it symbolizes anything (doesn't as far as I know), but because I think they look weird.
I would also stick with white gold or silver, as many girls don't like yellow gold. UNLESS she wears yellow gold already, don't buy her any. Take a cue from the things she already has.
IMO, necklaces like these are good:
http://www.zales.com/catalog/Product...hId=0&gnFlag=1
http://www.zales.com/catalog/Product...hId=0&gnFlag=1
http://www.zales.com/catalog/Product...hId=0&gnFlag=1
http://www.zales.com/catalog/Product...hId=0&gnFlag=1