QUARTZ (ROCK-CRYSTAL) 475 



leiisl common, nieasuve some metres in length and weigli sc\eral ]iundred\\'eights. The 



connnoncst are those of medium size, having the length and thickness of a finger or rathe 



more. 



Like all varieties of quartz, rock-crystal often contains enclosures of the most varied 

 kinds, which, on account of the extreme clearness of the substance in which they are 

 embedded, can be seen ^cry distinctly. Beside enclosures of foreign matter, rock-crystal 

 not infrequently contains ca^■ities which are either vacuous or filled partially or completely 

 with liquid. In the former case the remaining space is usually occupied by a bubble of gas, 

 which moves about in the cavity as the crystal is moved. Sometimes such cavities 

 containing liquid and an air-bubble are distinctly visible to the naked eye, but more often 

 they are of microscopic size. In the latter case they are often crowded togetlier in such 

 vast numbers, especially at the attached end of the crystal, that it ajjpears quite cloudy, 

 while the free end is perfectly transparent. In many cases the liquid, which fills these 

 cavities, may be easily proved to be carbon dioxide, but in other cases it is either water or 

 a solution of common salt. 



The enclosures of solid bodies, especially of crystals belonging to other mineral species, 

 are more important. They may be extremely small in size but jiresent in very large 

 numbers, so that they impart to the whole crystal a uniform colour, as in the case of green 

 prase and blue sapphire-quartz. On the other hand, these enclosures may be few in number 

 and large enough to be seen ■(\ith the naked eye, especially when they are conspicuously 

 coloured. Scales of green chlorite, for example, are often seen cither embedded in rock- 

 crystals or attached to the surface in such numbers that but little of the colourless quartz 

 substance can be seen, and the crystals appear of a green colour. Blades of white tremolite, 

 needles of green actinolite (Plate XVIII., Fig. 2), and of red or yellowish rutile, are often 

 enclosed in rock-crystal, and in many cases it is on the presence of these and other 

 enclosures, both fluid and solid, that the application of some varieties of quartz depends. 



Formerly rock-crystal was much used as a gem, and was cut as a brilliant, table-stone, 

 or rosette. A bi-ight vitreous lustre is imparted to it by polishing, and this can be slightly 

 increased by igniting the stone. Under favourable conditions cut stones show a certain 

 play of prismatic coloui's, and for this reason it was fashionable at one time to decorate 

 chandeliers, hanging lamps, candlesticks, &c., with prisms and pendants of rock-crystal. 



The mineral was at one time much used also for the fashioning of small articles of 

 more or less utility, such as crystal-balls, letter-weights, seal-stocks, &c. In the Middle 

 Ages all kinds of vessels, bowls, vases, and drinking cups, frequently so beautifully engraved 

 with pictured figures as to be transformed into veritable works of art, were carved out of 

 this substance. The art flourished most at the time when the manufacture of perfectly 

 clear and colourless glass was still in its infancy, and rock-crystal was the source from 

 whence material of this description could be most easily obtained. As the glass-making 

 industry developed and reached perfection, the art of working rock-cry.stal became forgotten, 

 for it was soon found that vessels equally transparent and finely finished could be made in 

 glass with much less labour than in rock-crystal. Articles carved out of rock-crystal are of 

 course more durable and not so readily scratched. Many art collections bear eloquent 

 testimony to the perfection which the art of working rock-crystal had reached among the 

 ancients, while in ancient writings there are to be found many records of historic vessels of 

 this sub.stance. 



At the present day rock-crystal is little used for gems or for the purposes described 

 above. On account of its comparatively great hardness and its resistance to chemical 

 reagents it is devoted rather to utilitarian purposes, such as the construction of lenses for 



