We are not finished describing diminishing fractions of rare earths,
but at least there are only a finite number of them.
Erbia (Jean Charles Galissard de Marignac’s oxide of erbium,
which he had separated, along with yttria and terbia
from Carl Gustav Mosander’s yttria)
was eventually found to have contained
erbium, thulium, holmium, dysprosium, scandium, ytterbium, and lutetium.
See also in The book of science:
Readings on wikipedia: