OBSIDIAN 447 



mass, but they sometimes give rise to the greasy Uistre already mentioned, and in a few cases 

 to a pecuHarly beautiful, reddish, silver-white, greenish-yellow or golden-green sheen, whif^h 

 shows up very effectively against the dark body-colour of the stone. Obsidian showing a 

 well-marked sheen is much valued, and is worth much more than the ordinary kind. 



In the remaining characters, obsidian preserves its resemblance to artificial glass. 

 Thus its specific gravity is low ; it varies between 2"3 and 2'5, rarely reaching or exceeding 

 2"6. In correlation with the fact that the mineral is of the nature of a lava we find the 

 chemical composition different for each occurrence. In every case, however, it contains 

 silicates of the alkalies and of aluminium, and of these alkalies a small amount of potash 

 is never absent. The amount of silica present varies between 60 and 80 per cent., and 

 the same wide variation in amount is shown by the other constituents. These are the same 

 as in ordinary glass, but are present in obsidian in different proportions. The following is 

 an analysis by Abich of a fine black obsidian from the island of Lipari : 



Per cent. 



Silica (SiOj) 74-05 



Alumina (AI2O3) . . . 12-97 



Ferric oxide (FsjOs) 2-73 



Lime (CaO) . . . 0-12 



Magnesia (MgO) . 0-28 



Potash (K2O) . 5-11 



Soda (NagO) , . 3-88 



Chlorine (CI) . 0-31 



Loss on ignition (water) . . 0-22 



99-67 



Again, obsidian is very brittle and breaks easily into sharp, angular pieces. On this 

 account care should be exercised both in the wearing of obsidian ornaments and also in the 

 cutting of the mineral. Its hardness is 5 to 5^, the same as that of window-glass ; obsidian 

 is therefore scratched by felspar, and still more easily by quartz, while it is itself scarcely 

 capable of scratching window-glass. Acids, with the exception of hydrofluoric, have no 

 action on this substance. It fuses before the blowpipe with intumescence, and then 

 solidifies to a grey, porous mass. 



Obsidian takes a high polish, and is worked as an ornamental stone in a variety of 

 ways. The kind which shows a coloured sheen is cut e7i cabochon, the sheen being displayed 

 to best advantage by this form of cutting. The ordinary black variety is used for mourning 

 iewellery of all kinds, brooches, sleeve-links, necklaces, bracelets, &c., but it is not often 

 used for ring-stones. It is usually cut with a plane or slightly convex surface, rarely with 

 a pronounced convexity. Faceted stones in the form of rosettes are frequently seen, as 

 are also faceted or spherical beads of obsidian. 



Obsidian was at one time much more extensively used than it is now ; at the present 

 time it is replaced by artificial glasses, which in depth and uniformity of colour and 

 silkiness of lustre are more than comparable with the naturally occurring glass. In the 

 case of cut stones it is impossible to decide at once whether the material of which they 

 consist is an artificial or a natural product. There is no difficulty, however, in dis- 

 tinguishing between these natural or artificial glasses and jet, another black mineral often 

 used in mourning jewellery. As we shall see later, jet is a variety of coal, and, like all 

 organic substances, is a bad conductor of heat. It therefore feels warm to the touch, 

 while obsidian and glass, being better conductors, feel cold. Other black stones which may 

 come into the market, black tourmaline and black spinel, for example, are heavier than 



