Rare earth is a general term for 17 elements of scandium, yttrium, and lanthanide series in IIIB group of the periodic table, and is often represented by  RE.

From the discovery of the rare earth element yttrium in 1794 to the discovery of the rare earth element promethium in nature in 1972, it took 178 years for people to find all 17 rare earth elements in nature. The luster of rare earth metals is between silver and iron, and the chemical activity of rare earth metals is very strong.

Rare earth is a name left over from history. Rare earth metals have been discovered one after another since the end of the 18th century.

At that time, people often called the solid oxide insoluble in water as soil, for example, called alumina as clay. Rare earths are generally separated in the state of oxides and are very rare, hence the name rare earths. The chemical properties of rare earth metals are very similar, so they coexist in minerals, but the chemical properties of scandium are quite different from other rare earths, and scandium is generally not contained in rare earth minerals.

Rare promethium was initially obtained from the fission products of uranium reactors, and the radioactive element 147Pm has a half-life of 2.7 years. Promethium was thought not to exist in nature until 1965, when a Finnish phosphate factory discovered traces of it while processing apatite.