The phone has been ringing off the hook for Lewis Black after China imposed export controls on tungsten, a niche metal mined by his firm that’s crucial to weapons manufacturing.

The CEO of North America’s Almonty Industries said his customers are in a “state of disbelief” following Beijing’s move on Tuesday, one of a suite of measures announced as a riposte to tariffs placed on Chinese goods by the Trump administration.

China accounts for about 80% of the world’s tungsten output, and there are concerns the government could add measures around tungsten scrap that would further constrict its availability. Almonty’s stock in Toronto has soared 41% over the last two days as investors price in scarcer supply of the super-dense material used in armor-piercing munitions, as well for engine parts and chip making.

“It’s the warning shot, because we cannot exist without it,” Black said in a phone interview from his base in New York on Thursday. “Our economy, manufacturing, defense, everything, is so dependent on it. And yet, Russia, China and North Korea have about 90% of the output.”

China has banned imports of tungsten scrap for a number of years, citing environmental concerns over how it’s processed. If it were to lift the embargo, it could suck in more supply and limit what’s left for other countries. That would create “a situation where it’s very difficult for my customers to compete with China,” said Black.

“The question is, how much will China tighten the screw to be heard?” he said. “I think the news was bad, but I think it’s going to get worse.”

China is the world’s biggest importer of the most heavily traded commodities like crude oil, soybeans and iron-ore, leaving critical minerals as one of the few areas where its dominance over supply give it leverage. Its latest export controls affect four other minerals in addition to tungsten that have applications in high-tech industries.

Beijing’s willingness in recent years to impose trade restrictions on critical minerals has forced companies in the US and its allies to seek alternatives to Chinese output. Almonty, which has operations in Portugal, is currently switching its domicile from Canada to the US. The firm is focused on expanding in South Korea, where it’s set to open a new mine in about two months that should in its first phase yield 2,500 tons of tungsten a year.

Almonty also produces molybdenum, another mineral targeted in this round of export controls. “It’s less dramatic,” Black said, because as a byproduct of copper mining there are more sources of the metal.