"This report is prepared pursuant to section 843 the Ike Skelton National Defense Authorization Act for Fiscal Year 2011 (Public Law 111 -383) and Senate Report 111 -201, accompanying S.3454, page 174. The Act requires the Secretary of Defense to submit a report to Congress on the supply and demand for rare earth materials in defense applications and Senate Report 111-201 requests discussion of national security issues related to rare earth materials in the defense supply chain. […] The group of elements called the 'rare earths' consists of 17 elements; 15 of these forming the chemical series called the lanthanides. Because of similar physical and chemical properties, scandium and yttrium are also considered to be rare earth elements (REEs). REEs are typically grouped into categories of 'heavy' or 'light.' Light rare earths are those with lower atomic number, such as lanthanum or cerium. Heavy rare earths include terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and yttrium. Despite the terminology 'rare earths,' some of these items are relatively abundant. Cerium is the 29th most abundant element in the Earth's crust and is three times as abundant as lead. Even the rarest REEs (such as thulium) are approximately 200 times more common in the Earth's crust than gold. However, all of the REEs (except for promethium) occur in low concentrations in the Earth's crust spread across various geographic regions. The Department of Defense (DoD) uses products containing rare earths in a wide variety of applications throughout its defense operations."