There have been times over the last 18 years when diamond prices...

There have been times over the last 18 years when diamond prices dropp substantially. I recall Rio decided some years ago to get out of diamonds altogether (was this somewhere around 2006/7?..). Not saying this is the only answer to your questions as there is likely more.

The following article is worth a read. Since De Beers became less dominant in the market pricing has been more market driven which means it goes down as well as up and follows market fortunes, it is after all a luxury good so in harder economic times will go down. https://astardiamonds.com/en/diamond-s-education/information/diamond-s-price-history.html