476 SYSTEMATIC DESCRIPTION OF TRECIOUS STONES 



spectacles, telescopes, and other optical instruments, lenses of this material being less likely 

 to be scratched than are those of glass. Its perfect transparency renders it useful for other 

 optical purposes, while it is used also for the manufacture of very exact weights to be 

 employed when extreme accuracy is required, for the pivot supports of various delicate 

 instruments, and for other similar purposes. 



The value of rock-crystal, like its use, has much diminished during recent times. It 

 depends upon the purity, transparency, colourlessness, and freedom from faults of the 

 material, such as enclosures of foreign matter, fissures, cloudy and coloured patches, and 

 anything, in short, which detracts from the uniformity of quality of the stone. The value 

 of any given stone depends also, of course, upon its size ; small pieces of perfect quality are 

 not uncommon, and for this reason the price of a cut ring-stone, even of the best quality, 

 scarcely ever exceeds ten shillings. Masses of considerable size and of good quality, on the 

 other hand, are not so easy to obtain, and the larger the size the higher relatively is the 

 price. 



Rock-crystal is very widely distributed, and is usually found, together with other 

 minerals, in the cracks and crevices of various ancient rocks, in which it forms druses, often 

 of enormous size. This mode of occiu-rence, taken in conjunction with the scarcely ever 

 failing presence of fluid enclosures, indicates very clearly that the mineral for the most part 

 has crystallised from an aqueous solution of silica. It would be impossible to mention 

 severally all the localities at which rock-crystal occurs ; a few typical occurrences only will 

 be here enumerated. 



Such in Europe are the high mountain peaks of the Tyrolese, Swiss, Italian, and 

 French Alps. The crystals, which are attached to the walls of crevices in granite, gneiss, 

 and other similar rocks, are collected and brought into the market by persons known in 

 Switzerland as " Strahlern." Their task is anything but easy, since the crystals almost 

 invariably occur in the highest and most inaccessible parts of these ranges. In the search 

 for crystal-bearing druses a " Strahler " is guided by the quartz- veins, which extend like 

 white bands across the faces of the rocks, and in which the druses are usually found. The 

 character of the sound given out when the vein is struck with a hammer indicates whether 

 there is likely to be a cavity, and therefore possibly a druse, at that particular spot. If 

 there is, the rock is broken open with a pickaxe or by blasting with powder or dynamite 

 and the crystals obtained. 



These cavities and the crystals contained in them are not usually of very large size. 

 Drusy cavities of enormous dimensions are, however, met with occasionally, and are known 

 as crystal-caves or vaults. Several hundredweight of crystals have been found in one such 

 cavity, and single crystals sometimes weigh a hundredweight or more. 



A celebrated find of this kind was made in the year 1719 in a crystal-cave on the 

 Zinkenstock, near Grimsel, in the Bernese Oberland. One crystal found here weighed 

 eight hundredweight, many weighed a hundredweight or more, and altogether 1000 hundred- 

 weight of crystals were taken from this gigantic druse. From a cave in the Vieschthal, 

 between Miinster and Laax, in Upper Valais, 1757 crystals, ranging in weight from 50 to 

 1400 pounds, were obtained. An occurrence of rock-crystal, only in comparatively small 

 druses, but often mentioned, is in the slightly auriferous quartz-veins of La Gardette, near 

 Bourg d'Oisans, in Dauphine (Department Isei'e), in the French Alps. From this locality 

 comes the group of crystals represented in Plate XVII. ; they always possess peculiar oblique 

 and unsymmetric terminations, which is characteristic, and distinguishes them from crystals 

 from other localities. At the beginning of the nineteenth century material from here and 

 elsewhere was cut in the lapidary works of Brianc^on on the Durance, Department Hautes 



