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 seized mines and built factories. Japan took note and invested in Australia. But the United States did little despite concerns about control of supplies.

From fighter planes to nuclear reactor rods and smartphones, rare earth minerals are vital to a wide array of products. As geopolitical and trade tensions rise, China’s dominance of both mining and processing means these niche metals have become a cudgel to use against opponents.