The concentration and quality of known Rare Earth Element (REE) deposits has been decreasing for decades.
Concentration determines the bar required for a mine to be profitable, and quality determines the available use case and market for the processed REE.
This is leading to a situation where the low hanging fruit as been picked. Ceteris paribus, Time and energy costs will increase in the future.
Connections
Rare Earth Elements Are Not Rare
Rare Earth Mines Can Take Decades To Reach Production
Reference
� The Elements of Power