There is no telling how the trade talks between the U.S. and China will pan out but China appears to be sitting at the table with a persuasive advantage: It has a global monopoly on samarium, a rare earth mineral that has magnetic properties and can withstand high temperatures. The U.S. military is dependent on the substance for its fighter jets.

China returns to trade talks with the U.S. holding, from its view, a strong hand: Its economy is bearing up better than expected. And it has an ace card: rare earths.

A new round of trade negotiations between the United States and China has started in London, with both sides trying to preserve a fragile truce brokered last month.