The rare earth elements are a relatively abundant group of 17 chemical elements (see Periodic Table) composed of scandium, yttrium, and the lanthanides. They were originally described as 'rare' because they were unknown in their elemental form, and difficult to extract from the rocks that contained them.
The principal economic sources of rare earths are the minerals bastnasite, monazite, and loparite and the lateritic ion-adsorption clays. The elements range in crustal abundance from cerium, the 25th most abundant element of the 78 common elements in the Earth's crust at 60 parts per million, to thulium and lutetium, the least abundant rare-earth elements at about 0.5 part per million. The elemental forms of rare earths are iron gray to silvery lustrous metals that are typically soft, malleable, and ductile and usually reactive, especially at elevated temperatures or when finely divided. All rare earth elements dissolve in acid to form triply charged ions in solution, and +3 is the default oxidation state.
The rare earths' unique properties are used in a wide variety of applications. For example, their ions are widely used as dopants in active laser media such as Nd3+:YAG (neodymium-doped yttrium aluminum garnet).