Promise Ring vs Engagement Ring
Promise rings are an engaged-to-be-engaged thing that is generally employed by the too-young-to-wed/Christian set. If you find a "promise ring" that you think your lady would like, get it and use it as an engagement ring. It's just a piece of jewelery. The phrase and hype, though, has become so well-known that you can actually be in the business of selling "promise ring for girlfriend" only and turn a profit.
This "promise ring" thing, as Engagement Ring Lite, is very recent. They're cheaper because they're marketed to be - if you can sell someone a promise ring and an engagement ring and one or two wedding rings, you've tapped the same customer 4 times. The price point is deliberately set below the arbitrary "two months' salary" designed by DeBeers in the first half of the twentieth century.
As rings themselves, they aren't going to be different or be identifiable as not engagement rings. If you see one you like, there's no reason not to use one as an engagement ring.
Here is some other opition:
1.To my understanding, a "promise" ring is just that. A promise to one day get engaged. An engagement is the actual thing, a real commitment and carries much more weight. To me, promise rings are a little "high school" but that may just be in my circle of peers. If you plan to marry her, go the estate jewelry route and get the real thing. Best of luck in theis big decision!
2.Where I come from, a promise ring is something a high school kid gives his girlfriend because they are too young to get engaged.
3.I think a promise ring is what you give your high school girlfriend right before you go off to college. It says, "I promise, one day you will get an engagement ring." And then you break up by Thanksgiving.
4.The promise rings seem much cheaper, but their style (to my aesthetically challenged eyes) is not obviously different.
The cost differential is the only real difference. A promise ring really doesn't mean much other than "I'm not ready to marry you yet but I have serious intentions of doing so at some point so here's this pacifier to signify my intent/keep you happy meanwhile".
And of course you're not going to spend a huge amount of money on said pacifier, even though it will resemble the engagement ring it supposedly foreshadows.
5.A custom promise rings says "I'll ask for your hand in marriage later, but I want a promise that you'll say yes now." I think it's a load of crap, but whatever floats the couples boat...
An engagement ring says "I'm asking you now: Want to get married?"
In practice though, there's not really that much of a difference other than the label.
Also, if you're worried about diamonds you could look into getting something Canadian. From my limited research (in which the girlfriend is helping because she knows what she wants better than I do), I've found that they're cheaper partly because they're not blood diamonds.