TOURMALINE 



365 



shown most conspicuously by the crystals lettered i, c, and d, there being many more faces 

 present on the upper than on the lower ends. Further, the hemimorphic development is 

 shown not only by the number and arrangement of the terminal faces, but also by that of 

 the prism faces. For example, in Fig. 70a, the prism has only three faces instead of six, 

 the number usually present on crystals belonging to the hexagonal system ; again, in 

 Figs, h to e, instead of a prism of twelve faces there are only nine, this being a combination 

 of a hexagonal with a trigonal prism. This feature is so characteristic of crystals of tour- 

 maline that they may be recognised with certainty by it alone, even when no terminal faces 

 are present. 



The faces of the prism are usually more or less deeply striated in the direction of their 

 length ; that is to say, parallel to the principal trigonal axis of the crystal, as is represented 

 in the figures. This striation is specially prominent when more than nine prism faces are 



6, o. d-. 



Fig. 70. Crystalline forms of tourmaline. 



developed, the crystals then having the appearance of cylinders, in which, however, a triangular 

 arrangement can still be detected (Plate XV., Figs. 8 and 9). The terminal faces are usually 

 smooth and not striated ; some, however, are rough and dull. The form of crystals of 

 tourmaline varies somewhat according to the locality at which they occur, but there is no 

 essential difference between them. Of the diagrams of Fig. 70, a represents a brown crystal 

 from Ceylon, h and c two green crystals from Brazil, d a red crystal from Shaitanka in the 

 Urals, and e a rose-red crystal from the island of Elba. These are all forms taken by 

 precious, transparent varieties of tourmaline suitable for cutting as gems ; such crystals are 

 usually small in size, rarely exceeding the length and thickness of a little finger. 



Tourmaline possesses no distinct cleavage. The fracture is uneven to imperfectly 

 conchoidal ; and, the mineral being very brittle, most crystals are penetrated by numerous 

 irregular cracks and fissures which tends to make them useless as gems. Tourmaline is just 

 sufficiently hard to scratch quartz, but is itself easily scratched by topaz ; its hardness is 

 thus between 7 and 1\, and may, as a rule, be taken as 7|. 



The specific gravity of tourmaline is not constant for all varieties, but ranges from 

 slightly over 3"0 to 3"2. All tourmalines sink in liquid No. 3 (sp. gr. = 3'0), though some 

 only slowly, and all float in pure methylene iodide. The density of the mineral varies with 

 its chemical composition, increasing with the amount of iron present, as may be seen from 

 the table of analyses with the con-esponding values of the specific gravity, quoted above. 

 Only in a few exceptional cases is the specific gravity slightly less than 3'0 or slightly over 

 3*2. It has already been pointed out that the greater the amount of iron present in any 

 given tourmaline the darker will be its colour and the more imperfect its transparency. 



