What are they?

Rare Earths are a moderately abundant group of 15 metallic elements known as the Lanthanide series (atomic numbers 57 through to 71) plus Yttrium (39).  Although Scandium (atomic number 21) is not a Rare Earth element, it is commonly included with the Lanthanides because of its similar properties.

The 15 lanthanides are represented by the single square of lanthanum in the main part of the periodic table and listed in a separate sub group below the main groupings.

They range in crustal abundance from cerium, the most abundant, at 60 parts per million, which is in fact more abundant than nickel or copper, to thulium and lutetium, the least abundant Rare Earth element at about 0.5 parts per million.