I'd guess that kind of thinking is reasonably common - anecdotally, my girlfriend and I are both in our early 20s and have been dating long enough that we've started discussing marriage. In one of our earlier discussions about it, I told her she shouldn't expect a mined diamond, because I'm morally opposed to the business and mining practices employed by diamond companies. She added the possibility of getting a recycled gemstone (which I didn't know about before!), agreeing that she wouldn't want a newly-mined stone.