When diamonds were discovered in South Africa at the beginning of the 19th century, the supply of diamonds was increased by a factor of thousands, threatening a crash in prices.
The solution was found by the De Beers cartel who created the idea of the mass market engagement ring:
“in 1880 almost no one in the U.S. owned a diamond engagement ring, by the 1920s it was expected that a middle-class bride would receive one. By the 1950s even laboring-class brides were expected to be able to display a diamond — thanks to the newfound formula of De Beers, according to which an engagement ring should cost a bridegroom two months of his salary, before taxes.”
Robert Proctor, Stanford Professor of the History of Science: The Agateer