368 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



colour, but in the other appear quite dark and only imperfectly transparent. The same 

 phenomenon may be observed in crystals from Paris, in Maine, U.S.A., which give gems of 

 a fine dark-green or of an unpleasing yellowish-green according to the direction in which 

 they are cut. Only in the cases where the crystal is pale in colour is it advantageous to cut 

 it so that the table facet is perpendicular to the prism edges, the resulting gem-stone being 

 thus rendered darker in colour than it would otherwise appear. With the requisite knowledge 

 it is thus open to the lapidary to very materially improve the appearance of a stone by 

 cutting it in a judicious manner. 



The optical refraction of tourmaline is somewhat feeble, but its birefringence is fairly 

 strong. There is a slight difference in the refraction of stones of different colours ; that is 

 to say, this character varies with the chemical composition. An increase in the amount of 

 iron present in the mineral is accompanied not only by an increase in the depth of its 

 colour, but also by an increase in the refractive index. The following are values for the 

 greatest and least refractive indices which have been determined for tourmaline of differejit' 



colours : 



Greatest Least 



refractive index. refractive index. 



Red tourmaline . . . 1-6277 1-6111 



Colourless tourmaline . . . 1-6366 1-6193 



Green tourmaline .... 1-6408 1-6203 



Blue tourmaline . . . 1-6530 1 -6343 



A very characteristic feature of tourmaline is the readiness with which it becomes 

 electrified. When rubbed it acquires in a very short time a comparatively large charge of 

 electricity, which it retains foi- some time ; and when subjected to changes in temperature it 

 becomes electrified still more readily. The behaviour of tourmaline in this respect is closely 

 connected with the hemimorphic development of its crystals. When a crystal is heated, one 

 end becomes positively and the other negatively electrified, but on cooling its polarity is 

 reversed. A faceted stone will, of course, behave in the same way, the portions corresponding 

 to the poles of the crystal being positively or negatively electrified. The largest electric 

 charges ai-e acquired by light-coloured crystals of transparent, precious tourmaline, which are 

 free from fissures. Under favourable conditions the charge may be so strong that shreds of 

 paper and similar light objects are energetically attracted. This pyroelectrical property 

 of tourmaline was observed in Holland at the beginning of the eighteenth century, when 

 the mineral first became known. Because it was observed to have the power, when cooling, 

 of attracting ashes to itself, the name aschtreMer, meaning in Dutch ash-drawer, was 

 bestowed upon it. No other precious stone resembling tourmaline in the smallest degree, 

 with the exception, perhaps, of topaz, is as strongly pyroelectric, so that this feature serves 

 to distinguish red tourmaline, for example, from ruby and other precious stones. The 

 electrical properties of the mineral may be demonstrated in a convenient manner by 

 dusting a mixture of red-lead and sulphur through a muslin sieve on to a cooling-stone, 

 when the sulphur will be attracted to the positively electrified portion of the stone and 

 the red-lead to the negatively electrified portion. 



Another of the characters of tourmaline is that it is unattacked by acids. The 

 colourless, pale green, and red varieties are infusible before the blowpipe, while the darker 

 varieties melt or run together and form a white to dark brown slag. 



As regards the mode of occurrence, tourmaline is confined almost entirely to older 

 crystalline rocks such as granite and gneiss. Other modes of occurrence, for example, in 

 granular dolomite at Campo-longo and in the Binnenthal, in Switzerland, are rare, and. 



