4U 



SYSTEMATIC DESCRIl'TION OF PRECIOUS STONES 



Fig. 75. Cry- 

 stalline form 

 ■of kyaiiite. 



prism, especially on the large broad face, and are represented in the figure. Kyanite occurs 

 in lamellar cleavage masses of considerable size, more frequently than in definite crystals. 

 These masses are sometimes of a fine blue colour; bhey show the perfect cleavage surfaces, 

 and are found in the same rocks as the well-developed crystals. 



Kyanite is neither very hard nor very brittle. A curious feature of this mineral is the 

 fact that its hardness is not the same on all faces of the same crystal, nor is it the same in 

 different directions on the same face. Thus in certain directions it is easily scratched by 

 felspar, while in others it is scarcely scratched by quartz; the hardness, 

 therefore, varies between 5 and 7. In no other mineral has there been 

 observed such a marked discrepancy between the hardness of different faces of 

 the same crystal. In consequence of the compai'ative softness of kyanite in 

 certain directions, special care should be taken in the wearing of cut stones. 



The specific gravity varies between 3"56 and 360, the latter value being 

 that of dark blue specimens of kyanite, which alone are used as gems. The 

 density of the mineral does not thus differ very much from that of the 

 heaviest liquid (sp. gr. = 3-6). Kyanite is infusible before the blowpipe, and 

 is unattacked by acids. 



As a general rule kyanite can only be described as being very translucent ; 

 perfectly transparent stones are not at all common ; and those which combine 

 transparency with a fine dark-blue colour are still more uncommon. In cutting, portions 

 -which are pale coloured or not perfectly transparent are removed as completely as possible, 

 and only material of a fine colour is utilised. 



The dichroism of the mineral is distinctly observable, but not strong ; it is best shown 

 by stones of a dark colour. The images seen in the dichroscope vary in colour between pale 

 and dark blue. The refraction of kyanite is considerable. The lustre of the crystal-faces 

 is of the vitreous type, while that of the cleavage faces is pearly. The natural crystal-faces 

 are not specially brilliant, but their lustre is increased to a certain extent by polishing. 



Both the table-stone and the step-cut are employed for kyanite ; but it is cut perhaps 

 most frequently en cahochon, when its fine blue colour is almost comparable to that of the 

 :sapphire. The two stones may be easily distinguished, however, for sapphire is both harder, 

 heavier, and more lustrous than is kyanite. Moreover, on most cut kyanites there are to be 

 ■seen fine, close-set lines, corresponding to the cracks mentioned above, running parallel to 

 ■each other across the surface. These lines, especially in stones cut en cabochon, sometimes 

 ■give rise to a band of feebly reflected light, similar to the appearance shown by cymophane. 

 Small cleavage cracks in the direction of the perfect cleavage are also not infrequently 

 ■observed. The most valuable stones are those which are pure and transparent and of a 

 ■deep colour, but they are never worth very much, and their use, at any rate in Europe, is 

 very limited. 



Kyanite is moderately abundant and is distributed somewhat widely, but material of 

 :gem-quality is found at only a few localities, and then in small amount. Among others 

 ■may be mentioned Monte Campione, near Faido, on the southern slopes of St. Gotthard, in 

 •Canton Tessin. Definite crystals, though only pale blue in colour, are found here in 

 abundance ; they have the chemical composition quoted above. Associated with them is 

 a«ddish-brown staurolite, to be considered presently ; both are found in a matrix of white, 

 finely scaled mica-schist. 



Massive kyanite, sometimes containing portions of gem-quality, occurs also in mica- 

 schist in the Zillerthal and the Pfitschthal in the Tyrol. At the summit of Yellow Mountain, 

 near Bakfirsville, in North Carolina, U.S.A., fine dark blue crystals occur in a white quartz- 



