China has suspended almost all exports since April 4 of seven kinds of rare earth metals, as well as very powerful magnets made from three of them. The halt has caused increasingly severe shortages that threaten to close many factories in the United States and Europe.

Why are these metals so needed, why has China stopped exporting them and, crucially, what happens next?

What are rare earths?

There are 17 types of metals known as rare earths, which are found near the bottom of the periodic table. Most of them are not actually very rare — they are all over the world, though seldom in large enough ore deposits to be mined efficiently.

They are called rare because it is very difficult to separate them from one another. Breaking the chemical bonds that bind them in nature can require more than 100 stages of processing and large quantities of powerful acids.

Why does China control so much of the rare earth supply?

China mines 70 percent of the world’s rare earths. Myanmar, Australia and the United States mine most of the rest. But China does the chemical processing for 90 percent of the world’s rare earths because it refines all of its own ore and also practically all of Myanmar’s and nearly half of U.S. production.