In western culture, still today I think,majority of men would be buying and majority of women would be receiving diamonds. It would be interesting if this affects answers. Would a buyer go for more practical cheaper option while receiver goes for more expensive traditional options? Or a different split completely , or none? I think it'd be a fascinating exercise.
I found majority of my men friends argued it's the same diamond without the ethical concerns if you go man-made, while majority of my women friends chose natural (reasons included social pressure, the story, and so on).
Anecdotes aren't proof. But perhaps there is something to looking at this from a gendered (proxy for giving vs. receiving?) lens.
If the professor is making an argument that one choice is clearly the rational choice, and then highlights differences in how different groups make that choice, then they are directly implying that some groups are more irrational than others.
This phenomenon has to be more localized than Western culture (the worldwide diamond consumption hints at it being US-specific?) - I for example don't know anyone who even contemplated buying one for proposing. Granted, this is purely anectotal, but over extended family and workplace colleagues this includes a bunch of milieus.
My family in Europe doesn't have the whole engagement ring concept.