Colour brown, varying in zones by twinning, with distinct pleochroism. Hardness; sp. gr. varies from 4·4 to 6·0, being about 5·5 to 5·6 for fairly pure material. Double refraction negative, 2 E = 70-75°. Acute bisectrix nearly coincident with _c_ axis, plane of the optic axes _b_, (010). The mineral is insoluble in acids, readily soluble in fused potassium hydrogen sulphate. Before the blowpipe it is almost infusible; it dissolves in the fused borax bead, rapid cooling causing separation of crystals. If a bead containing zirconia be heated until the borax is partially volatilised, zirconia crystallises on cooling in tetragonal crystals, isomorphous with those of rutile.[91] [91] Nordenskiöld, _Pogg. Ann._ 1861, ~114~, 625; for tetragonal zirconia see also Troost and Ouvrard, _Compt. rend._ 1888, ~102~, 1422. The mineral was discovered in 1892 by Hussak and L. Fletcher independently. The former, who obtained it from the pyroxenite sand of São Paulo, South Brazil, believed it to be a tantalo-columbate, and called it Brasilite. Fletcher found it in a gem-gravel from Rakwana, Ceylon, and named it Baddeleyite. An analysis by Blomstrand of Hussak’s mineral showed it to be identical with the Ceylon mineral, and Hussak withdrew his name and accepted Fletcher’s. It has recently been found[92] in a corundum-syenite, near Bozeman, Montana, U.S.A. [92] Rogers, _Amer. J. Sci._ 1912, [iv.], ~33~, 54. The mineral now comes on the market in commercial quantities; pure zirconia almost entirely free from iron can be obtained by leaching with acids. The pure oxide is extraordinarily refractory, and promises to be of great use for crucibles, furnace linings, etc. (_vide_ p. 324). ~Rutile.~--Titanium dioxide, TiO₂, occurs crystallised in nature in the three minerals Rutile, Brookite, and Anatase (Octahedrite), which therefore form a trimorphous series. They are all stable minerals, though rutile appears the most stable, being occasionally found in pseudomorphs after the other two. The family is remarkable in that it is not unusual to find two of them occurring together--an uncommon phenomenon with polymorphous minerals. Rutile often contains small quantities of iron and chromium. The ferriferous varieties are distinguished as Nigrine, which is black, with 2-3 per cent. ferric oxide, and Ilmenorutile, with up to 10 per cent. of ferric oxide, and specific gravity up to 5·13. Crystal system--tetragonal, holosymmetric; _c_ = 0·6442; (001) ∧ (101) = 32° 47´. Common forms--prisms _a_ {100}, _m_ {110}, and _l_ {310}; pyramids _e_ {101}, _s_ {111}, and many others. The basal pinakoid _c_ {001} is very rare. Habit, prismatic, with vertical striations; or in slender needles. Twinning very common and varied; usually on the cassiterite law--twin plane _e_ (101)--forming the knee-shaped twins, and irregular rosettes by repetition, and many contact twins. Contact twins on the law--twin plane _v_ (301) are less common.