America’s only rare earths mine has fielded calls from anxious companies since China responded to President Donald Trump's tariffs by limiting exports of seven heavy metals.

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.

Chinese export limits are energizing a push for more domestic mining of these minerals, which are used in electronics and military applications.