What is a rare earth, regarded by some as an oxide of the supposed element gadolinium, by others as only a mixture of the oxides of yttrium, erbium, ytterbium, etc called?

gadolinia — A rare earth, regarded by some as an oxide of the supposed element gadolinium, by others as only a mixture of the oxides of yttrium, erbium, ytterbium, etc. (Webster's 1913.)