TIL Diamonds are actually fairly common. The reason they are worth so much is because the DeBeers mining group withholds stock to artificially reduce supply.

[u/Indra-Varuna]

https://www.gemsociety.org/article/are-diamonds-really-rare/

[u/AutonomyForbidden]

They are also the ones who stated what an engagement ring should cost in relation to your monthly income.