TRANSPAREiNT STONES 



569 



colour of tourmaline is usually very dark and inclined somewhat to green, while that of 

 sapphire is usually a pure blue ; moreover, tourmaline is very much more strongly dichroic 

 than is sapphire. 



Kyanite almost invariably exhibits a series of fine straight cracks running in one 

 direction ; it is not perfectly transparent as a rule, and, compared with sapphire, has little 

 lustre. It remains suspended in or sinks slowly to the bottom of liquid No. 1, in which 

 sapphire readily sinks. Diamond of a deep blue colour is extremely rare ; it is recognisable 

 by its specific gravity, hardness, and single refraction. 



Cordierite is not a gem ordinarily met with in the trade ; it is distinguished by its low 

 specific gravity and strong dichroism, with the characteristic shades of colour. 



Glass and haliynite are both singly refracting and not dichroic, but the specific gravity 

 of the former is considerably higher than that of the latter. 



5. VIOLET STONES. 



Apatite and fluor-spar are scarcely ever used as gems : their specific gravity and lack 

 of hardness distinguish them from other violet stones, and the difference in optical 

 refraction distinguishes the one from the other. Axinite is included in the same division ; 

 it is characterised by specially strong dichroism, and its colour is not pure violet, but 

 markedly brownish in shade. 



Of the stones included in this group, the one met with most commonly is the true 

 amethyst, which constitutes Division V., and is readily distinguished from the members of 

 the first division by its low specific gravity and feeble dichroism. 



Almandine is distinguished from the other stones included in Division I. by its colour, 

 which is better described as red inclined to blue than as pure violet. It differs both from 

 " oriental amethyst " and from true amethyst in that it is singly refracting, and from the 

 former in its hardness. " Oriental amethyst," amethyst, and spinel are, perhaps, most 

 conveniently distinguished by the presence or absence of dichroism, which is strongest in 

 the first, feeble in the second, and absent from the last. " Oriental amethyst " and spinel are 

 capable of scratching quartz, but amethyst, of course, is not, and spinel diff^ers from the 

 other two in being singly refracting. The only well-marked difference between spinel and 

 almandine is one of density ; the former remains suspended or slowly sinks in liquid No. 1, 

 while the latter quickly sinks. The slight difference in hardness may enable one to 

 discriminate between these stones, but in certain cases it is difficult to distinguish one from 

 another. 



Glass differs in hardness from all the violet stones ordinarily used, being readily 



