Beijing’s ban on exporting six valuable minerals highlights America’s reliance on China for many raw materials that Washington has designated “critical.”

MP Materials , which owns the only U.S. rare earths mine, said on Thursday it has stopped shipping the critical minerals to China for processing after Beijing boosted import tariffs to 125% in response to U.

A US producer of rare earth metals stopped shipments to China after Beijing placed export controls on similar materials crucial for defense and technology manufacturing.