The rare earths are a relatively abundant group of 17 elements that are so chemically similar that 15 of them occupy a single row and column in the periodic table.

They occur as oxides, or “earths” in the old chemical jargon, and they are not really so rare. The elements range in crustal abundance from the 25th most abundant element in Earth’s crust at 60 parts per million (similar to copper) to 0.5 part per million.

They were called “rare” because they are dispersed, and the similarity of their chemical properties makes their separation difficult. The first discovery was at a quarry in the village of Ytterby, Sweden, in 1787, where the ytteria earth was identified.

In 1794 Jons Jacob Berzelius analyzed a similar deposit from another quarry in Sweden and called it ceria, after Ceres, the largest asteroid in the solar system. By 1803 chemists had isolated the element cerium.

It took 40 years for researchers to determine that ceria and ytteria contained other elements as well. In 1842 four more elements were isolated.

Another 30 years passed with no new developments, but by 1879 the flame spectroscope was sensitive enough to identify the weak spectrum of other elements contained in the ores.

During the 1940s the Manhattan Project adapted the methods used to separate plutonium-239 and neptunium from uranium, thorium and actinium produced in nuclear reactors to develop chemical ion exchange procedures for separating and purifying the rare earth elements.

Today the elements are separated industrially from each other by a long series of solvent extractions.

The rare earth elements have many uses in the modern world, especially in electronics.

Many of the devices we take for granted today such as cellphone and laptop computers would be five times larger and heavier without the neodymium magnets in speakers and hard drives.

Neodymium magnets also play a role in electrical generators and motors where a high magnetic density is required, as in wind generators and servomotors.

Flat-screen displays, fluorescent lights, metal alloys for batteries and lightweight structures, lasers, wind generators and electric vehicles are just a few of the items that use various rare earth elements.

China produces over 97% of the world’s rare earth supply, although it has only 37% of proven reserves.

Since 2009 China has reduced its export quotas by more than 40% as it has halted production of some major mines that produce almost 40% of its total rare earth production.

China’s restrictions on exports along with increased demand have ramped up the search for alternative sources in Australia, Brazil, Canada, South Africa, Greenland and the United States where mines were closed when China undercut world prices in the 1990s.

A 2010 U.S. Geological Survey report found that the United States had 13 million metric tons of rare earth deposits. The Mountain Pass Mine in California, which was the world’s largest producer until the 1980s, has been revived in recent years.

The USGS is also actively surveying southern Afghanistan for rare earth deposits. The USGS study team mapped 1.3 million metric tons of source rock, about 10 years of supply at current demand levels. The Pentagon estimated its value at about $7.4 billion.