Some of these reasons aren't terribly surprising.
Regarding 1) and 2), while the details aren't easy to confirm, it can be argued that diamonds are a fad that has persisted over time. What purpose does the diamond serve?
a) It tells the world that the woman is engaged/married. You know, as if a woman's marital status should define who she is.
b) The size/quality of the diamond serves to tell the world that the woman is engaged/married to a man who hails from a certain income bracket (or worse, often how rich his parents are). While not universal, there is a large percentage of women (and men for that matter) that like to show off their *economic status*.
Regarding 10), while movies are hardly a reliable source of information, you do see a few movies where white-collar and international crime involves the exchange of diamonds for some service or good.
Regarding 5) and 6), this applies to other precious rocks as well. Heck, countries will throw native people off their soil just for oil and farming.
Regarding 4) and 7), this applies to so many other good we import (Nike shoes are notorious for human rights violations), so I wouldn't put it past diamond-related businesses to use the same tricks.
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I can't speak for who I eventually commit to, but I'm sure she'd rather me pull my weight (you know, changing stinky diapers, cleaning the house, tending to the kids, etc; you know, do stuff *that matters*) than put a month (or three) of salary into a piece of bling and then treat her like property or a slave (for household work and sex).
Maybe I'll just waste one week's salary...
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[EDIT]: If this point wasn't clear, it's worth noting that diamonds are hardly the only thing people buy that are wildly overrated. Men and women alike often make purchases that have less to do with utility and more to do with being popular/attractive and overall trying to "fit in," and many of them also happen to involve workers that have their basic human rights violated.
I could unleash a long-winded rant about materialism, but I just don't have the energy to fight that fight again in a forum (yet).