It is not so much that used diamonds are worth less (although they might decline in value without provenance to prove they are natural or if they are chipped) but the huge markup on retail jewellery. It's easy for any member of the public to buy and sell gold at close to market price, it's much harder with diamonds.

> It is not so much that used diamonds are worth less (although they might decline in value without provenance to prove they are natural or if they are chipped) but the huge markup on retail jewellery.

Precisely.

And on top of that some jewelry stores are worried that customers would consider a below wholesale offer to be insulting, so they often refuse to buy piece back at all.