Precious Stones: Olivine - Sphene Page 273



	270 PRECIOUS STONES. unknown. It is found in good specimens in Brazil near Villa Rica; in the Alps on Monte Campione in the St. Gothard district in schist; in the Tyrol; on Mount Greiner in the Zillerthal; and near Baskerville, in North Carolina, in the United States. The colours often resemble Sapphire, but Cyanite is softer and less dense. 403. Euclase. Euclase is a rare mineral, occurring in crystals of a green or greenish blue colour, often transparent, and having a vitreous lustre. It is doubly refracting, though only in a feeble degree, the greatest and least indices for yellow light being 1'671 and 1"652. The dispersion is also small, but there is a distinct pleochroism. It is easily electrified by friction, and on strongly heating gives off water. The specific gravity is 3'05 to 3"10. It is brittle, shows a con-choidal fracture, and has three cleavages parallel to the three primary crystal forms. It occurs in short monosym-metric prisms vertically striated, the crystals often being highly modified. In composition it is a hydrated beryllium aluminium silicate, H₂0, 2 BeO, A1₂0₃, 2 Si0₂. It occurs in chlorite schist near Novas Minas, in Brazil, with yellow Topaz; also near the Sanarka River, in the Urals, in gold-bearing alluvium, with Topaz and Corundum. It has also been found in very small crystals in the Alps. It maybe distinguished from Beryl (variety Aquamarine) by being of higher specific gravity, and more strongly dichroic, and from blue Topaz by being less dense and more dichroic.