Question

What are rare earth elements?

Answer 1

naturally occurring oxides of the lanthanide metals

Answer 2

http://en.wikipedia.org/wiki/Rare_earth_element

Answer 3

Common Properties of the Rare Earths

These common properties apply to both the lanthanides and actinides.

The rare earths are silver, silvery-white, or gray metals.
The metals have a high luster, but tarnish readily in air.
The metals have high electrical conductivity.
The rare earths share many common properties. This makes them difficult to separate or even distinguish from each other.
There are very small differences in solubility and complex formation between the rare earths.
The rare earth metals naturally occur together in minerals (e.g., monazite is a mixed rare earth phosphate).
Rare earths are found with non-metals, usually in the 3+ oxidation state. There is little tendency to vary the valence. (Europium also has a valence of 2+ and cerium also a valence of 4+.)