The rare-earth elements (REE), also called rare-earth metals, or rare earths, are a set of 17 nearly indistinguishable lustrous silvery-white soft heavy metals. The 15 lanthanides (or lanthanoids), along with scandium and yttrium, are usually included as rare earths. Compounds containing rare-earths have diverse applications in electrical and electronic components, lasers, glass, magnetic materials, and industrial processes. Rare-earths are to be distinguished from critical minerals, which are materials of strategic or economic importance that are defined differently by different countries, and rare-earth minerals, which are minerals that contain one or more rare-earth elements as major metal constituents.
The term "rare-earth" is a misnomer, because they are not actually scarce, but because they are found only in compounds, not as pure metals, and are difficult to isolate and purify. They are relatively plentiful in the entire Earth's crust (cerium being the 25th-most-abundant element at 68 parts per million, more abundant than copper), but in practice they are spread thinly as trace impurities, so to obtain rare earths at usable purity requires processing enormous amounts of raw ore at great expense.
