I and most of my friends (late 20s to early 30s) have non-diamond engagement rings. The consensus is that diamonds are outdated, exploitative, and scammy. I also think they’re not as pretty/eye-catching as my lab-grown emerald.
If that is true, then buying something more expensive than the usual thing doesn't invalidate any of the arguments against diamond engagement/wedding rings.
If emeralds are cheaper, then while I'd rather have the emerald anyway (it's prettier to my eyes), I can't see any person (male or female) preferring the cheaper jewelry.