Should i buy my girlfriend a necklace?

That's a really cute idea. The necklace has the same idea as a promise ring, but isn't quite as serious. Be aware, though, that she may be mad that you don't feel serious enough to have gotten her a promise ring. Personally, I would much rather get the necklace, but if a girl is wanting a promise ring, it might be best to get her one if you're prepared for it...