Gadolinite n. : A mineral of a nearly black color and vitreous luster, and consisting principally of the silicates of yttrium, cerium, and iron.

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Taxonomy
Gadolinite, free encyclopedia, mineral, cerium, variable element, element composition, search, vitreous luster, formula, prominence, crystals, collectors, desire, fractures, conchoidal, element yttrium, gadolinium, second element.

Defination from the web:
Gadolinite was named in 1800 for Johan Gadolin, the Finnish mineralogist-chemist who first isolated an oxide of the rare earth element yttrium from the mineral in 1792. ... However, gadolinite does not contain more than trace amounts of gadolinium. ...