A bunch of people in my social circle have gotten married in the last year or so, and some of them didn't do the diamond ring thing that people often do. I don't know if rings with other kinds of rocks were involved or what. I'd be happy enough to see the social custom of diamond-giving fall by the wayside, both for blood diamond reasons, and because there are a bunch of better ways to use the money.
My tentative plan for the time of my life associated with diamond-ring-giving is to make some kind of big splashy charitable contribution out in the Third World that I can put my wife's name on -- maybe a village school named after her, or a scholarship for poor farm kids. (A US dollar goes really far in places like India.) I'll give her a pretty ring with some far less valuable stone that I can somehow connect to the charitable act, perhaps with the name of the school inscribed on the inside or something like that. Then I'll obnoxiously go around telling all my single friends what I did, with hopes that people will show me up by making a bigger charitable contribution than me when it's their time. That, after all, is how you change social customs for the benefit of mankind.
For all this to work out, I'd have to find myself a woman awesome enough to appreciate this sort of thing. But that's part of the plan anyway.