Diamonds have a slightly more complex role than just being conjured from thin air by marketing.

Premarital sex is not, popular opinion to the contrary, a new discovery. In most societies we know of, however, men prefer to marry women who have never slept with anyone else. This creates a problem. Unmarried women are reluctant to have sex for fear that it will lower their ability to find a suitable husband, and as a result unmarried men have difficulty finding women to sleep with.

One traditional solution to this problem is for unmarried couples to sleep together on the understanding that if the woman gets pregnant the man will marry her. This practice was sufficiently common in a number of societies for which we have data that between a quarter and half of all brides went to the altar pregnant.

One problem with this practice is that it creates an opportunity for opportunistic breach by the man, the strategy of seduce and abandon familiar in folk songs, romantic literature, and real life. That problem can be reduced by converting the understanding into an enforceable contract. Under traditional common law, a jilted bride could sue for breach of promise to marry. The damages she could collect reflected the reduction in her future marital prospects. They were in fact, although not in form, damages for loss of virginity.

Starting in the 1930's, U.S. courts became increasingly reluctant to recognize the action for breach of promise to marry, with the result that between 1935 and 1945 it was abolished in states containing about half the population. This created a problem for women who wanted to engage in premarital sex but did not want to end up as single mothers in a society where that status was both economically difficult and heavily stigmatized.

The solution they found was described in "Rings and Promises," an ingenious article by Margaret Brinig. The practice of a man giving his intended a valuable diamond engagement ring is not, De Beers' ads to the contrary, an ancient custom. Data for diamond imports in the early part of the century are not very good, but Brinig's conclusion from such information as she was able to find was that the practice only became common in the 1930's, peaked in the 1950's and has since declined.

Her explanation was that the engagement ring served as a performance bond for the promise to marry. Instead of suing, the jilted bride could simply keep the ring, confiscating the posted bond. The practice eventually declined not because of further legal changes—at present no states recognize the action for breach of promise to marry—but as a result of social changes. As pre-marital sex became more common and virginity of less importance on the marriage market, the risk of opportunistic breach, and thus the need for a bonding mechanism, declined.

http://www.daviddfriedman.com/Laws_Order_draft/laws_order_ch...

So there was an opening market for something that could be construed as both romantic and serve a strictly financial purpose. Diamonds could fit that bill, and DeBeers deftly latched onto it and milked it hard.

I think marketing has power to shape and inform already existing needs and desires, but it has extremely limited power to create need where none exists. That is, you can plausibly make me want a BigMac over a Whopper, but you can't make be want a burger if I'm already full.