I think the geological formations that were guaranteed to contain diamonds only became common knowledge in the late 1990's or early 2000's, yes? I recall reading an article about some researchers testing their theory by buying a chunk of land in Canada, and proving they were right.

People thought that was the beginning of the end for de Beers, and between that and better synthetics that seems to be how things played out.

It's also a product that's not required for day-to-day.