Crystals commonly prismatic, terminated by _c_. Faces rough and coarse; lustre vitreous to greasy, seen only on freshly-broken surfaces. Brittle. No cleavage. Fracture conchoidal to splintery. Hardness 6¹⁄₂-7; sp. gr. 4·0-4·5. Colour black, greenish- and brownish-black; green and transparent in flakes. The crystalline variety has strong positive birefringence, with the plane of the optic axes parallel to (_b_), the plane of symmetry; the amorphous variety is of course isotropic. The brown variety shows very distinct pleochroism, _i.e._ the colour as seen by transmitted light varies with the direction in which the light traverses the crystal; the green kinds have much weaker pleochroism. Gadolinite is of common occurrence in the pegmatite veins of the Scandinavian granite. It was first found in a felspar quarry on the island of Ytterby, near Stockholm, by a Lieutenant Arrhenius[24]; it is also found, together with a large number of other rare earth minerals, at Fahlun. It occurs in Norway on the islands of Hitterö and Malö, and in Germany in the Riesengebirge and the Harz. Probably the largest deposit is that in Texas, at Barringer Hill, near Bluffton, on the west bank of the Colorado River, Llano County, now owned and worked by the Nernst Light Company of Pittsburg; in 1904 a mass of very pure gadolinite weighing 200 lb. was found here.[25] [24] _Vide_ Geijer, _Crell’s Chemische Annalen_, 1788, ~1~, 229. [25] See _U.S. Geol. Survey_ (_Minerals_), 1904, 1213. In the same place a decomposition product of gadolinite was discovered by Hidden and Mackintosh in 1889. They named it Yttrialite or Green Gadolinite. It contains no beryllium, and twice as much silica as the parent mineral, and approximates to the formula R₂O₃,2SiO₂, where R₂O₃ is chiefly yttria oxides; it is thus similar in composition to the newly found scandium silicate, Thortveitite (_q.v._). It is amorphous and massive; and is often found in continuous growth with gadolinite. Pieces up to 10 lb. in weight have been obtained.