The tentative deal between the United States and China may represent a retreat from the worst-case scenario of a total collapse of trade between the world's two biggest economies, but it creates more problems than it solves.

Rare earths are among the most critical raw materials on the planet, deeply embedded in the technologies that underpin modern life. Yet few people have heard of them or know what they do.

A tentative deal reached in London gives Beijing continued leverage in trade negotiations.