De Beers also single handedly invented the idea that diamonds should be given for an engagement. Prior to the 20th century,
diamonds were not especially coveted or considered valuable because of their relative abundance, and were mostly used for industrial purposes. De Beers set about to create a
market other than
industrial uses for diamonds so they came up with the whole engagement idea.
In the 20s and 30s they paid movie studios to add scenes featuring the now familiar man on his knees presenting a diamond, which previously had nothing to do with courtship. They chose that method because it was thought that if the man were expected to 'surprise' the woman with a diamond, it would be preferable to the already engaged couple going to pick out a gem together, in which case they might level-headedly decide that the money was better spent on something else.
What a business model! Monopolize the distribution of a marginally valuable product, create a market for it from scratch, and raise armies to act with extreme vengeance against anyone who may challenge the cartel.