De Beers started it during the great depression when people begin hoarding hard currency commodities like gold and diamonds. Prior to this but contributing to, an influx of mines in South Africa caused the price of diamonds on the world market to drop. De beers then created an ad campaign to convince people that diamonds for the inevitable conclusion of courtship. They managed to begin controlling the supply and demand and price of diamonds on the world market. Prior to the 1920s it was not common to give diamond rings.