Diamonds are not scarce, are not investments (they depreciate), and are no more "forever" than any other rock. Their popularity as stones in engagement rings dates to the 1930s with DeBeers' very effective marketing campaign.
On the economics of diamonds, the biggest marketing scam in history by the world’s most successful and enduring cartel:
We covet diamonds in America for a simple reason: the company that stands to profit from diamond sales decided that we should. De Beers’ marketing campaign single-handedly made diamond rings the measure of one’s success in America. Despite its complete lack of inherent value, the company manufactured an image of diamonds as a status symbol. And to keep the price of diamonds high, despite the abundance of new diamond finds, De Beers executed the most effective monopoly of the 20th century. Okay, we get it De Beers, you guys are really good at business!
De Beers believes that a fellow should spend two month's income on a diamond. Wiki has a brief history of engagement rings and wedding rings.
Carpe Diem recommends fake diamonds, and real love and reliable companionship.