Rare earth metals
Rare earth metals, also known as "rare earth elements" or "rare earth minerals" are a group of 17 metals that are widely used in technology and industry. In the Periodic Table of Elements, they occupy the block of two rows of elements located below the main body of the chart, plus lanthanum (element 57) and actinium (element 89).  There are two blocks of rare earths, the lanthanide series and the actinide series. 
They are not in fact particularly rare, but can be difficult to extract because they rarely exist in pure form and instead are mixed in with other minerals underground.  They are often found mixed with radioactive materials or other harmful substances, making extraction potentially hazardous to the environment.
Extraction has become easier over the years, however, as new processes such as ion-exchange and solvent extraction came to be used to more quickly produce pure, low-cost rare earths.
China has dominated world trade in these metals because of its willingness to intensively mine rare earths and to deal with the radioactive or acidic byproducts involved. The Chinese government has heavily subsidized state-owned mining companies and allowed them to operate without strict environmental oversight. The U.S. has large deposits of its own but still gets 92 percent of its rare earths from China. In March of 2012, President Barack Obama joined the European Union and the Japanese government in filing a complaint with the World Trade Organization (WTO) about China's manipulation of the rare earth metals market.