system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why diamonds are as expensive as they are if they aren't really that rare.
gpt
De Beers.

Basically up until fairly recently the De Beers Diamond Trading Company controlled the *vast* majority of the diamond supply.  They collected around 80-90% of the diamonds that were dug up in a given year, but instead of selling them all out on the open market  they put them in storage.  

By keeping their diamonds in storage and only allowing a certain amount of diamonds hit the open market, they limited the quantity supplied out to the open market, inflating the price of the diamonds.  For a long time De Beers executives couldn't come to the US because they would be arrested for violating anti-monopoly laws. 

At this point De Beers doesn't control as much as they used to, but diamonds are still expensive because the price is what we expect diamonds to be.  Diamonds have always have been expensive.  Why would they suddenly become cheap?

It also doesn't hurt that they're aesthetically pleasing and shiny.  Also becoming the cultural norms for engagement rings haven't hurt either.