Rare earth elements are a set of 17 elements that are difficult to come by; while they are relatively abundant, they don't tend to congregate in large amounts in the earth's crust. This makes it hard to sustainably, economically, and cleanly mine them. In recent years the work has largely been taken over by China, where regulations are more lax, explains Popular Science:

China has supplied 91 percent of U.S. consumption of rare earths between 2005 and 2008, and continues to represent the world's largest rare earth exporter. But the Chinese have warned that their own domestic industry appetite for rare earths may eventually force them to stop exporting--an action that would leave the U.S. high-tech industries crippled without other readily available supplies.

Without these rare earths, high-tech and green industries could grind to a halt. And China knows it too; the country made its point by temporarily stopping shipments to Japan earlier this year after a diplomatic incident. U.S. scientists have warned Congress that the significant demand for these minerals for use in high-tech and green technologies (like wind turbines, computer parts, and hybrid car batteries) could lead to shortages. Technology Review says that in fear of future shortfalls, many companies are already looking to replacement options:

In response to China's dominance in rare-earths production, researchers are developing new materials that could either replace rare-earth minerals or decrease the need for them. But materials and technologies will likely take years to develop, and existing alternatives come with trade-offs.

Others are looking into re-opening rare earth mines in the United States, including some recently found deposits to the tune of 13 million metric tons, so perhaps rare earths haven't peaked just yet, USGS representative told Tech EYE:

"This is the first detailed assessment of rare earth elements for the entire nation, describing deposits throughout the United States,” says Marcia McNutt of the US Geological Survey. "Although many of these deposits have yet to be proven, at recent domestic consumption rates of about 10,000 metric tons annually, the US deposits have the potential to meet our needs for years to come."

The only problem left is how to extract them safely, cleanly, efficiently, and most of all, quickly.