When my wife and I got engaged 25 years ago, we agreed to forgo the engagement ring. That was at a time when buying a diamond was tantamount to supporting apartheid. Quite a few years later, she told me she’d like a ring, but that it was a waste of money to get a diamond. So I got a nice CZ ring on eBay.
If you want to know the social history of diamonds, I recommend this long 1982 article from The Atlantic:
http://www.theatlantic.com/magazine/archive/1982/02/have-you-ever-tried-to-sell-a-diamond/4575/