From "Diamonds are Bullshit," 2013.05.19 by Rohin Dhar of Priceonomics.com. Emphasis mine.

And so, in 1938, De Beers turned to Madison Avenue for help. They hired Gerold Lauck and the N. W. Ayer advertising agency, who commissioned a study with some astute observations. Men were the key to the market:
Since “young men buy over 90% of all engagement rings” it would be crucial to inculcate in them the idea that diamonds were a gift of love: the larger and finer the diamond, the greater the expression of love. Similarly, young women had to be encouraged to view diamonds as an integral part of any romantic courtship...

...Lauck needed to sell a product that people either did not want or could not afford. His solution would haunt men for generations. He advised that De Beers market diamonds as a status symbol:
”The substantial diamond gift can be made a more widely sought symbol of personal and family success — an expression of socio-economic achievement.”
…
"Promote the diamond as one material object which can reflect, in a very personal way, a man’s … success in life."

This article also contains an interesting discussion of the price economics of diamonds, which due to their complicated and subjective valuations end up being remarkably poor investment vehicles. Interesting stuff.