420 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



represented on the scale by 5. While the hardness of dioptase is much less than that 

 of emerald, the specific gravity is higher, being 3'28. The two minerals are therefore 

 readily distinguished by means of these characters, but usually dioptase can be recognised 

 at a glance on account of its dark colour, imperfect transparency, and the presence of 

 numerous small cracks. 



The principal locality for dioptase is the hill of Altyn-Tiibe on the western slopes of 

 the Altai Mountains in the Kirghiz Steppes in Siberia. This is a limestone hill, and is 

 traversed by numerous crevices which are filled up for the most part by calcite ; it is on 

 this calcite tha,t the dioptase in sparing amount is found: the mode of occurrence is 

 illustrated in Plate XV., Fig. 4. A few comparatively large crystals of dioptase have also 

 been found in some of the gold-washings in the Yeniseisk Government, where they occurred 

 loose and in a more or less rounded condition. For a long time the occurrence of large 

 crystals of dioptase was thought to be confined exclusively to Siberia, and the use of this 

 mineral as a gem was limited to Russia, Persia, and countries in the near neighbourhood. 

 This is still the case at the present day, although latterly fine crystals have been found in 

 other parts of the world, especially in the French Congo. 



In spite of the fine colour of dioptase, its deficient hardness and transparency prevent 

 its extensive use as a gem ; for the same reasons the price of the stone is always low. 



CHRYSOCOLLA. 



The amorphous mineral chrysocoUa is closely allied to dioptase in chemical composition, 

 being a hydrous silicate of copper, but containing more water than dioptase. Its hardness, 

 H = 2 — 4, is low, sometimes less even than that of calcite ; the mineral is lacking also in 

 transparency, but in spite of these drawbacks it is sometimes cut as an ornamental stone on 

 account of its fine green or blue colotn:. This is especially the case with stones from the 

 copper mines of Nizhni-Tagilsk in the Urals. In the Allouez mine, near Houghton, in the 

 copper region of Lake Superior, North America, the mineral occurs mixed with quartz ; 

 this affords a much harder material more suitable for cutting, and from it are obtained 

 magnificent bluish-green stones half a square inch in area. 



GAENIERITE. 



Gamierite or noumeaite has certain characters in common with chrysocolla, and may 

 therefore be mentioned here. Like chrysocolla it is opaque, amoi-phous, of a fine green 

 colour, and softer than calcite. Chemically it is a hydrous silicate of nickel. It occurs as 

 large masses in the island of New Caledonia, Australasia, where it is extensively worked as 

 an ore of nickel, and where some few stones are cut for ornamental purposes. 



