From the U.S. Geological Survey news release "Going Critical":

"Although currently no U.S. Government-wide definition exists, broadly speaking, if a vital sector of the economy requires a mineral in order to function, that mineral would likely be deemed “critical".

Rare earth elements are hardly the only critical minerals. They’re not even the only minerals critical to the high-end technology sector. Another mineral vital to the functioning of your smart phone is gallium, a soft, silvery metal. Without gallium, the semiconductors that power smartphones and data-centric networks would not be possible. Unlike rare earths, gallium is not a common metal in the Earth’s crust, but it does occur regularly alongside aluminum in a mineral known as bauxite. One of gallium’s other claims to fame is that it has such a low melting point that it will melt if held in your hand.

Another critical mineral is manganese, which is an important metal alloying ingredient. Without manganese, stainless steel would not be possible. In addition, it helps other metals resist rust and corrosion, such as iron and aluminum. Manganese is a fairly common element in the Earth’s crust, and exists in many concentrations easily mineable."

Learn More:

• Tracking the Global Supply of Critical Materials (Webinar), American Geosciences Institute
  2018 webinar detailing efforts from the U.S. and European Union to monitor the global production, trade, and usage of critical raw materials; identify and quantify vulnerabilities in supply; and support co-operation on production, recycling, and substitution of key mineral resources.

• Underpinning Innovation: The Science and Supply of America's Critical Minerals and Materials (Webinar), American Geosciences Institute
  2016 webinar addressing efforts being taken on the federal level to ensure a steady supply of critical minerals and materials.

• REE Background (Webpage), Wyoming State Geological Survey
  A precise definition of rare earth elements (REEs) and in-depth discussion of how common REEs are, worldwide REE production, and current REE production.
   
• Rare Earths Statistics and Information (Website), U.S. Geological Survey
  Portal for information on annual production, price, and use of rare earth elements since 1997, plus links to further information.
   
• Minerals, Critical Minerals, and the U.S. Economy (Report), National Research Council
  A 2007 report that provides background on what defines a "critical" mineral and proposes a strategy for defining and tracking critical minerals at the federal level.
   
• Critical Materials Institute (Website), U.S. Department of Energy
  Website of the Critical Materials Institute (CMI), an energy innovation hub operated by the Ames Laboratory for the U.S. Department of Energy. The CMI brings together people from government, academia, and industry, to develop new technologies that make better use of critical minerals or use alternative resources to decrease supply risk.