I mention this because my wife is extremely unassuming, and socially and environmentally conscious, and she still couldn't shake the diamond thing. She was 100 percent aware of this and it bothered her but in the end decided it mattered to her nevertheless. We ended up getting a small polar bear diamond: the metalwork (by a local metal artist) is really more what you'd notice about her ring.
I'm still not sure what it was, nor is she. If I had to guess now it's that for her, once you decide to have a gem, it's all the same issues at some level, so why not use the one that has personal meaning for you, whatever that is, for whatever reason? We looked into synthetics, but at the time they weren't actually available. I think if there weren't other options she might have went with a repurposed stone? Maybe no stone?
Moissonite is a great option, one I wish were more used. We learned about it from the metalsmith who made our rings, and I noticed recently she had stopped actively promoting them.
I think to see a shift from diamonds, these types of conversations just need to happen repeatedly more than everyone thinks. The idea needs to be suggested when people are very young that different symbolic expressions of marriage are fine, and that what's important is whatever is meaningful to the members of the couple. By the time people are adults the schemas might already be indelibly fixed.
But then again who knows what marriage and commitment will be like for my daughter. I don't envy the young in certain ways.