446 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



SODALITE. 



This is another blue mineral belonging to the same group. Like haiiynite, it occurs 

 as grains enclosed in lapis-lazuli, and has a limited use as a gem. It occurs also as larger 

 grains and as crystals belonging to the cubic system. It is usually dull or quite colourless, 

 but specimens are sometimes met with of a pronounced blue colour and closely resembling 

 lapis-lazuli in appearance. The material most frequently cut for gems is that found in a 

 syenitic rock, loose blocks of which lie about on the surface of the ground at Litchfield, 

 Maine, in the United States, where the stone is worn on account of its national origin. 

 Similar material is yielded by rocks of the same kind at Ditro, in Transylvania, and at 

 Miask, on Lake Ilmen, in the Ural Mountains. Fine large masses also occur on the Ice 

 river, in British Columbia, and at Dungannon, in Hastings County, Ontario. Sodalite was 

 used as an ornamental stone by the ancient inhabitants of the Bolivian tableland, beads of 

 blue sodahte and of fluor-spar, together with arrow-heads of quartz and obsidian, having 

 been found by the traveller, Alfons Stiibel, in the ruins of Tiahuanaco, on Lake Titicaca, 

 one of the most ancient cities of South American civilisation. Sodalite is not known to 

 occur in the region, so that the material for these objects must have been brought from 

 some locality unknown. 



OBSIDIAN. 



Obsidian is not a simple mineral, but a glassy lava or volcanic glass, belonging to the 

 rhyolite group of rocks. It is sometimes worked for ornamental purposes, and is known to 

 lapidaries by several names, among which are lava, black lava-glass, volcanic glass, and 

 ^' glass-agate." 



Obsidian, like artificial glass, is perfectly amorphous, and, therefore, optically singly 

 refracting. It has a perfect conchoidal fracture, such as is seen in glass, and exhibits the 

 vitreous lustre characteristic of that substance, though the lustre may sometimes incline to 

 the greasy type. Typical obsidian is thus wonderfully similar to ordinary glass, and differs 

 from it markedly only in colour and transparency. Obsidian may be black, grey, brown, 

 yellow, red, green, and sometimes blue, but is always deeply coloured, and, because of this, 

 almost perfectly opaque. Very thin splinters alone are transparent and at the same time 

 colourless, or nearly so. Obsidian may be of one uniform colour, or it may be patchy and 

 streaked with various coloin-s. Thus a variety from North America, known as " mountain 

 mahogany," is streaked with brown and grey, and when cut shows a grain like that of 

 mahogany. Obsidian of a uniform black colour is more important and more widely 

 distributed ; this variety, when perfectly homogeneous and uniformly coloured, is admired for 

 the silkiness of its appearance, and is cut for ornamental purposes. 



Very frequently obsidian is not uniformly coloured, nor even apparently homogeneous in 

 structure, but contains embedded in it crystals, sometimes of appreciable size ; such 

 specimens are useless for cutting. Those which appear to be homogeneous are not so in 

 reality. When examined under the microscope they are seen to contain large numbers of 

 minute, spherical, or elongated cavities (so-called vapour pores), minute crystals of all 

 kinds, and other enclosures. These are too small to affect the beauty of the stone in the 



