The Rising Influence of Rising Affluence

[Biowatch]

The rare-earth elements, also called the rare-earth metals or (in context) rare-earth oxides, or the lanthanides (though yttrium and scandium are usually included as rare-earths) are a set of 17 nearly-indistinguishable lustrous silvery-white soft heavy metals. Scandium and yttrium are considered rare-earth elements because they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties, but have different electronic and magnetic properties…

Despite their name, rare-earth elements are relatively plentiful in Earth's crust, with cerium being the 25th most abundant element at 68 parts per million, more abundant than copper. All isotopes of promethium are radioactive, and it does not occur naturally in the earth's crust; however, a trace amount is generated by decay of uranium 238.

https://en.wikipedia.org/wiki/Rare-earth_element

Although lanthanides are usually viewed as the rare earth elements, actinides also fall into that category.

https://www.mpcomagnetics.com/blog/rare-earth-properties-lanthanides-and-actinides/

It’s my understanding that this person was convinced that he’d isolated and purified each rare earth element, but ion exchange chromatography demonstrated that none of his samples were completely pure.

Charles James, a chemistry professor at the University of New Hampshire from 1906 to 1928, was an internationally-recognized expert in rare earth chemistry. In a laboratory in Conant Hall, James devised novel fractional crystallization techniques for separating rare earth elements, which were widely adopted by other chemists. James used his method to separate large amounts of ytterbium, previously considered to be a single element, into two elements now known as ytterbium and lutetium.

https://www.acs.org/content/acs/en/education/whatischemistry/landmarks/earthelements.html