480 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



represented in Plate XVIII., Fig. Sa, and a cut-stone of paler colour in Figs. Sb and c, of 

 the same plate. 



The colouring of smoky-quartz is due to the presence of a volatile organic substance 

 containing carbon and nitrogen, which can be distilled oft' as a turbid liquid. The distilla- 

 tion is accompanied by a smell of burning, which is noticeable even when a dark coloured 

 specimen, that is to say, one containing much colouring matter is broken, or when two 

 fragments of the same are vigorously rubbed together. On igniting smoky-quartz, or on 

 merely raising its temperature to 200° C, the organic colouring matter is completely 

 destroyed, and the mineral becomes indistinguishable from rock-crystal. At a low 

 temperature the brown colour becomes yellow like that of citrine, another variety of 

 quartz. Probably not a little of the yellow quartz which comes into the market is none 

 other than " burnt " smoky-quartz. 



With the exception of colour, all the characters of smoky-quartz are identical with 

 those of rock-crystal. The similarity in the habit of the crystals, and their mode of 

 occurrence, is specially striking, and we are, therefore, justified in stating that smoky-quartz 

 is simply rock-crystal of a brown colour. This is not always the case, for we shall see later 

 that amethyst, a violet-coloured variety of precious quartz, difi"ers from rock-crystal not 

 only in colour but also in structure. 



All that has been said with regard to the crystalline form (Fig. 85) of quartz, and its 

 occurrence in crevices in the gneissic and granitic rocks of the Alps and at other localities, 

 together with the existence of very large drusy cavities containing hundredweights of the 

 finest crystals, applies equally well to smoky-quartz, and need not be repeated. 



The most remarkable occurrence of smoky-quartz in large crystals of deep black 

 morion was discovered in August 1868, in a crystal cave near the Tiefen glacier in Canton 

 Uri, Switzerland. The cave was in weathered granite, and out of it was collected in a 

 very short time 300 hundredweights of crystals, 200 hundredweights of which were 

 beautifully transparent and suitable for cutting, and the remainder for museum specimens. 

 Among the latter are some of remarkable size, the largest of which are now exhibited in 

 the Berne Museum. Several of these crystals are distinguished by special names: the 

 "Grandfather" is 69 centimetres long, 122 in circumference, and weighs 133^ kilograms. 

 The " King " is rather thinner and less heavy, but longer and better preserved than any ; 

 it is 87 centimetres long, 100 centimetres in circumference, and weighs 127J kilograms. 

 The two smallest of the half-dozen crystals preserved at Berne are named "Castor "and 

 "Pollux"; they are 72 and 71 centimetres in length and 65 and 62| kilograms in 

 weight. Each of these crystals was attached to the matrix at one end, but in the case of 

 one large crystal, which has perfectly developed faces at both ends and on all sides, it is 

 impossible to see the point of attachment, although it must originally have grown attached 

 to the matrix just like the others; this is 82 centimetres long, 71 in circumference, and 

 weighs 67 kilograms. Another crystal found at the same time and place is preserved in 

 the Mineral Department of the British Museum (Natural History) ; it weighs 299 pounds 

 and is just over a yard long. 



Compared with this Alpine occurrence of smoky-quartz all others are unimportant. 

 It is met with accompanying topaz, beryl, and specially amethyst, at Mursinka, in the 

 Urals, and with beryl and topaz in the neighbourhood of Nerchinsk, in Transbaikalia. 

 In the form of pebbles it is found with pebbles of rock-crystal in the Rhine ; and in the 

 same way at Alen^on, in Normandy, and again in the gem-crystals of Ceylon. 



In Scotland it is found on Cairngorm, a mountain on the borders of Banftshirc and 

 Inverness-shire, and ou this account a brown transparent quartz is often referred to in 



