A simple visit to an obscure factory by Chinese President Xi on Monday is all it took to raise the specter that China could be contemplating cutting off supply of critical materials to the U.S. and potentially crippling large swathes of its industries. Also, fueled by political innuendo in Xi’s recent call for a new “Long March” in reference to a key founding tenet of the Chinese Communist Party, speculators are growing increasingly wary of Chinese export restrictions to the U.S., including rare earth minerals. As the world’s largest producer, the Middle Kingdom has a vice-like grip on rare earths...