America’s only rare earths mine heard from anxious companies soon after China responded to President Donald Trump’s tariffs this month by limiting exports of those minerals used for military applications and in many high-tech devices.

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.

The U.S. relies on China for about 70% of its rare earth imports. New Chinese restrictions on exports of the key minerals highlight why presidents from both parties have been trying to kickstart domestic production.