TRANSPARENT STONES 



571 



ruby," and Bohemian garnet; the last two do not differ essentially and both belong to the 

 sub-species pyrope. These, together with spinel, are distinguished from ruby by their 

 single refraction, and from each other by differences in colour. Almandine is purplish-red 

 (inclined to blue), "Cape-ruby" and Bohemian ganiet are both dark blood-red with a 

 yellowish tinge, and hessonite is pale yellowish-red. The different varieties of garnet are 

 characterised also by not unimportant differences in density. The variety of spinel known 

 as " ruby-spinel " is very similar in colour to ruby, but is easily distinguishable by the means 

 indicated already. On the other hand it is often difficult to distinguish spinel from garnet, 

 especially in the case of the reddish-yellow spinel (rubicelle) and hessonite, which agree in 

 the character of their refraction, in the absence of dichroism, in colour, and very closely in 

 specific gravity ; it is sometimes possible, though difficult, to distinguish cut stones by their 

 difference in hardness. There is less difficulty in distinguishing rough stones, since spinel 

 always occurs in octahedra, while hessonite scarcely ever takes this form, and spinel is 

 infusible and hessonite easily fusible before the blowpipe. The fusibility of hessonite and 

 almandine distinguishes them also from the infusible pyrope. 



Red tourmaline and topaz in colour are very similar to each other, and also sometimes 

 to ruby, but each of the three differs very definitely from the others in specific gravity, and 

 a further distinction is afforded by the fact that one of the two images shown by topaz 

 in the dichroscope is distiiictly yellow, this not being the case with red tourmaline or 

 ruby. 



In the second division diamond is distinguished from topaz by its single refraction 

 and lack of dichi'oism, and these same features serve to distinguish fluor-spar from tourmaline, 

 both of which are included in Division III. 



The specific gravity of fire-opal is always much lower than that of any other stone in 

 this group. It is singly refracting, always rather cloudy, and sometimes shows a play of 

 colours like that of precious opal. Glass is distinguished from the stones of this group in 

 the usual way ; the most convenient distinction between glass and opal is that of the 

 difference in specific gravity, the density of glass being scarcely ever less than 2"6. 



8. REDDISH-BROWN STONES. 



Staurolite is rarely met with, it is seldom perfectly transparent and always very darkly 

 coloured. Tourmaline and almandine of a reddish-brown colour are rarely cut as gems. 



