Iron is arguably the most important, and also degrades - past, as I understand it, the point at which it can be recovered by low-tech processes. The longer it takes to bootstrap a second industrial revolution the harder it would become.
Yeah this is the issue with metals and likely the largest barrier to a post collapse society. Likely ironworking would end up looking like pre-industrial japan where iron was veryextremely valuable and of relatively low quality compared to the west due to the problems with smelting it from iron sands.
This is why I propose long term technological advancement without a second industrial revolution.
