> at a fraction of the cost
That might also be the reason. If something is cheaper, it feels inferior.
Also with diamonds it's probably a factor that the fiancé is expected to present serious intents with a deeper monetary investment. "Look honey, it's an ethical, clean diamond and only cost 1/20th of a dirty one" sad, but feels wrong.
Edit: I also remember a guy at dinner party bragging about buying a 5000EUR ring, it goes both ways.