A month after China announced it would limit exports of germanium and gallium, both required for semiconductor production, its overseas shipments of the materials dropped to zero.

Although Beijing has approved some export licenses since then, the restrictions are a stark reminder that China has a powerful weapon in the escalating trade war. In order to restrict China’s access to key military technology, the United States, Europe and Japan restricted sales of chips and chipmaking equipment to the country.

It is still too early to tell how strict the restrictions will be. Xiaomeng Lu, director for geotechnology at Eurasia Group, said that if China blocks large amounts of exports, it will disrupt the supply chain for the immediate consumers.

It is estimated that China produces both elements to a near monopolistic level. The US Geological Survey (USGS) reported that it produced 98% of the world’s gallium last year and 68% of refined germanium.

In spite of the fact that there are alternatives available to the United States and its allies, an independent supply chain for gallium and germanium processing could require a “staggering” investment of more than $20 billion, according to Marina Zhang, an associate professor at the University of Technology Sydney. The development process could take years.

Considering the environmental effects of their extraction and mining, gallium and germanium refineries can’t be constructed overnight, she wrote in July.