Promise rings - well, you can also call them commitment rings - that doesn't sound quite as "young" does it? It's exactly the same thing. More often than not, a couple will buy a matching pair. Over here in Denmark, a couple will buy a set of commitment rings - and use them again as their wedding bands. Either that or they use the commitment rings as a kind of "substitute" to an engagement. It means the same thing - it just means that both partners get to wear a ring, not just the woman. But when all is said and done, does it really matter how he shows his commitment to you? No. Is it absolutely nocessary for a woman to get "bling" in her lifetime? No. Just because a diamond is "traditional", there are other ways of doing it. It could be that he doesn't have the money to buy you the bling that you want. Perhaps you could discuss with him a way to show your commitment to one another, that reaches a compromise?
I understand where you're coming from - I'm fairly set on the same idea - but I can also understnad why he's so hurt. But better that he knows how you really feel, than finding out later and thinking he's been taken for a ride. Show him a lotta luvin' and I'm sure he'll come around.