I have a friend who shall remain nameless, lets just call him Class Act. Anyway, Class Act's girlfriend of about 2 years was hinting around at marriage and wanting to get engaged and wanted some type of promise ring and commitment from him. So, Class Act decided that he would buy a promise ring, and after giving it to her and she was showing it off to some friends at a social gathering, like it was something to be proud of and some major commitment, Class Act decided it was time to tell her, along with everyone else, that it wasn't a promise ring, it was a promise to get her a promise ring. It was fucking small enough that it could have come from a CrackerJack box. They are still together but I don't think she even wears it anymore and I don't think you could even tell what it was when she did.
Just wait until you are ready to buy an engagement ring and ask her to marry you, don't waste your money on some miniature polished terd for $300-$500.