Rare earths, on the other hand, have narrower supply chains and are often more difficult to extract, requiring more cumbersome processes to separate them from other minerals (as Lecoq de Boisbaudran learned). The United States has just one operational rare earth mine, in Mountain Pass, Calif., which produces around 15 percent of global rare earths.

MP Materials said on Thursday it had stopped shipping rare earth concentrate to China, in response to retaliatory tariffs and export controls, sending its shares down 6.3% in afternoon trading.

China is retaliating against U.S. tariffs by restricting exports of rare earth minerals. Minerals expert Gracelin Baskaran explains why these little-known-elements are so vital to defense and tech.