China has managed to hit the U.S. where it hurts the most in response to the Trump administration’s tariffs, with restrictions on global exports of certain rare-earth elements that the U.S. has little ability to produce or refine.

China’s dominance of both mining and processing means these niche metals have become a cudgel to use against opponents.

China has stopped exporting rare earth minerals and magnets after U.S. President Donald Trump raised tariffs on Chinese goods to 145%, according to The New York Times. Shipments have been halted at all Chinese ports while Beijing works on new rules that could block sales to certain companies — including American defense contractors.