American artist Lee Gainer has created a variety of images of engagement rings that can be bought for two-months's salary. This synopsis is for a truck driver.Anyone who's seen the excellent HBO series, Rome, knows that the Romans were the first to give rings as a sign of engagement: at the beginning of Season 2, badass Titus Pullo, in proposing to his long-unrequited love, shy Eirene, because he spent all of his money on drinking and gambling, ties a blade of grass around her finger.  The Romans also smeared dirt on each other's faces. 

In the Middle Ages, as diamonds were seen to withstand both fire and steel, a platinum diamond engagement ring was given by princes to princesses as a sign of the unbreakable vows of marriage.  It wasn't until the discovery, in 1870, of the Kimberley Diamond Mine, that diamonds became "not so rare a gem after all."  The price of diamonds fell rapidly, and, as anyone with a really old grandmother knows (mine's 93), the birthstones phenomenon started.  At this time, it was far more common to give a potential wife an engagement ring with her birthstone as opposed to a diamond ring, because diamonds were seen as cheap and vulgur.

Fast forward to 1947.  Platinum has been banned for personal use, since the military needed it to kill people in World War II.  De Beers responds by introducing the "Diamond is Forever" campaign.  Jewelers capitalize by instructing customers that one-month's salary is the proper amount to pay for a platinum diamond engagement ring.  (This was later upped to two-months's salary.)  Men don't want to be seen as cheap and miserly on the Day of Reckoning, so they buy it.  Mad Men are hired to spread the idea through all of society: the 1953 film Gentlemen Prefer Blonds has pop culture icon Marilyn Monroe singing "Diamonds are a Girl's Best Friend" as a boon to De Beers et al.  Am I cynical?  I'm the guy that, when asked what I'd do if I won the lottery, replies "I'd put it in the bank.".

As the Diamond Wholesale Corporation puts it: 

This is where the tradition of the diamond engagement ring really started, all from an advertising campaign . . . So you see, buying a diamond engagement ring isn't really a popular ancient tradition.  It's more a combination of dreaming of being a princess, a compelling marketing campaign, and compelling Hollywood glamour that ultimately promotes diamonds as the only jewels with which to furnish your loved one as a sign of engagement.

Undoubtedly, the folks at De Beers are marketing geniuses.  My question is: why hasn't Ford or Chrysler thought of this?  Start a marketing campaign where not giving your son or daughter a new car upon graduating high school is seen as cheap and miserly.  That should take care of the looming bankruptcy that plagues American automakers.