DIOPTASE  , a rare

mineral species consisting of acid copper orthosilicate, H2CuSiO4, crystallizing in the parallel-faced herniahedral class of the rhombohedral system . The degree of sym- metry is the same as in the mineral phenacite, there being orly ar axis of triad symmetry and a centre of symmetry . The crystals have the form of a hexagonal prism m terminated by a rhombohedron r, the alternate edges between these being sometimes re-placed by the faces of a rhombohedron s . The faces are striated parallel to the edges between r, s and in . There are perfect cleavages parallel to the faces of a rhombohedron which truncate the polar edges of r: from the cleavage cracks internal reflections are often to be seen in the crystal, and it was on account of this that the mineral was named dioptase, by R . J . Hauy in 1797, from Sioirrciav, " to see into." The crystals vary from transparent to translucent with a vitreous lustre, and are bright emerald-green in colour; they thus have a certain resemblance to emerald, hence the early name emerald-copper (German, Kupfer-Smaragd) . Hardness 5; sp. gr . 3.3 . The mineral is decomposed by hydrochloric acid with separation of gelatinous silica . At a red heat it blackens and gives off water . The fine crystals from Mount Altyn-Tube on the western slopes of the Altai Mountains in the Kirghiz Steppes, Asiatic Russia, line cavities in a compact limestone; they were first sent to Europe in 1785 by Achir Mahmed, a Bucharian merchant, after whom the mineral has been named archirite .

More recently, in 189o,

good crystals of similar habit, but rather darker in colour, have been found with quartz and malachite near Komba in the French Congo . As drusy crystalline crusts it has! been found at Copiapo in Chile and in Arizona . Dioptase has occasionally been used as a gem-stone, especially in Russia and Persia; it has a fine colour, but a low degree of hardness and the transparency is imperfect . (L . J .