Gadolinite is a mineral of a nearly black color and vitreous luster, and consisting principally of the silicates of cerium, lanthanum, neodymium, yttrium, beryllium, and iron with formula: (Ce,La,Nd,Y)2FeBe2Si2O10. Called gadolinite-(Ce) or gadolinite-(Y) depending on the prominence of the variable element composition(namely, Y if it has more yttrium, and Ce if it has more cerium).
Gadolinite is fairly rare, but it forms attractive crystals that some collectors desire. Its hardness is between 6.5 and 7, and its specific gravity is between 4.0 and 4.7. It fractures in a conchoidal pattern. The mineral's streak is grayish-green.
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. The rare earth gadolinium was also named for him. However, gadolinite does not contain more than trace amounts of gadolinium.