570 



DETERMINATION OF PRECIOUS STONES 



scratched by a steel point ; it is distinguished both from " oriental amethyst " and from 

 true amethyst by its single refraction. 



6. LILAC AND ROSE-COLOURED STONES. 



Of the stones of this group, the first three are commonly met with, tourmaline is less 

 common, and the others make only rare appearances in the precious stone market. " Balas 

 ruby" exhibits not infrequently a milky cloudiness. Topaz and tourmaline both show 

 differently coloured images in the dichroscope. Topaz is distinguishable also from ruby 

 and spinel by the difference in specific gravity, and the two last named differ from each 

 other both in the character of their optical refraction and in the fact that dichroism is 

 feeble in the one and absent in the other. Glass is easily recognisable by its lack of 

 hardness and by its single refraction. 



7. RED STONES. 



Of these, red fluor-spar, red diamond, and fine opal are all rare. 



Division I. embraces the garnets — almandine, hessonite or cinnamon-stone, " Cape 



