With obscure names like lutetium and dysprosium, rare earths are used in everything from iPhones and magnetic resonance imaging machines to missile guidance systems. More recently, demand has been propelled by the green tech that is helping to cut carbon emissions.

China, with whom Trump has threatened a trade war, is the world's largest producer of rare earths and many other critical minerals. Both the U.S and Europe have sought to reduce their dependence on Beijing.