For years, the Chinese government has worked to control the export of rare earths, a group of metals used in an array of products, as common as semiconductors and lights. Now, in its trade war with the United States, China is moving to limit the market for these metals even further, which could have disastrous consequences for American manufacturing and military power. So, what exactly are these metals, and why are they so important? What are rare earths? There are 17 types of metals known as rare earths, which span the periodic table and are crucial to industries like technology, energy and transportation. With names like terbium, praseodymium and dysprosium, the metals are important ingredients for some of the most advanced technologies. Rare earths can be sorted into two kinds: heavy and light. Heavy rare earths have a greater atomic weight and are typically more rare, meaning they sell in smaller quantities and are prone to shortages. Light metals, by contrast, have a lesser atomic weight. The two most important are neodymium and praseodymium, which …