Artificial scarcity and massive amounts of marketing. De Beers has stock piles of diamonds and trickles them out at a steady pace to keep the prices from dropping.

A little over 60 years ago, they paid celebrities to wear diamond rings. Specifically chose engagements and wedding rings as the thing to associate with diamonds, because people lose all common sense when something is attached to romance and love. The image of a woman breathless when the box is opened and the guy proposes, goes straight into the emotional brain because we pick up cues about how we should feel about things, based on other peoples reactions. They paid media outlets to talk about the ‘new diamond trend’.

Lastly, after convincing people they had value and are also an ‘investment’, they turned around and said ‘a diamond is forever’. Well then, who wants to buy a used engagement ring from someone who’s marriage didn’t work out? That’s no symbol of eternal love anymore. So they hampered the resale market as well.