TRANSPARENT STONES 567 



doubly refracting and spinel singly refracting. Colourless spinel is very rare and is scarcely 

 ever met with cut as a gem. Zircon and corundum are commoner, and not infrequently are 

 substituted for the diamond ; the latter, however, is distinguished by the fact that it floats 

 in liquid No. 1. Zircons and « white sapphires" of some size may be distinguished by an 

 exact determination of their specific gravities ; and another difference between the two 

 stones lies in the hardness, for the latter readily scratches a smooth surface of topaz while 

 zircon does not. Moreover, colourless zircon has a brilliant adamantine lustre, while the 

 lustre of corundum is of the vitreous type. Finally, cut colourless zircons are nearly always 

 small, at the most no larger than a pea, and there is often a perceptible reddish tinge about 

 them, left after heating the yellowish-red hyacinth, which is never exhibited by colourless 

 corundum. 



Division II. embraces topaz and diamond ; the specific gravity is practically identical, 

 but topaz is doubly refracting and diamond singly refracting ; and there is also an enormous 

 difference in hardness, diamond scratching topaz very easily and deeply. Moreover, the 

 strong adamantine lustre of diamond is very characteristic, and the magnificent play of 

 prismatic colours, shown by cut stones, still more so, so that it is scarcely possible to mistake 

 diamond for topaz or for strass. The last-named is readily scratched by a steel point 

 or marked by an aluminium pencil, and also feels warmer to the. touch than does diamond. 



Coloui-less tourmaline is the only stone embraced by the third division, and is of very 

 rare occurrence as a gem. 



Phenakite and beryl, the two stones which constitute division IV., can be distinguished 

 only by an exact determination of their specific gravity : if performed with the aid of 

 methylene iodide the smallest fragments of material will suffice. Characteristic of phenakite 

 is its specially strong lustre, by which it is distinguished from colourless beryl, which is 

 rarely cut as a gem. 



Rock-crystal falls into division V. and cannot, therefore, be mistaken for other and 

 heavier stones. 



From many of the stones in this group glass can be distinguished by the fact that it 

 is singly refracting. So, indeed, also is opal, but the transparent, glassy variety of opal known 

 as hyalite is practically never used as a gem. It may be recognised by the fact that it is 

 capable of scratching glass, though it itself is easily scratched by a steel point. The feeble 

 anomalous double refraction always shown by water-clear opal also serves to distinguish it 

 from glass. 



2. GBBENISH-BLUB (SEA-GBEEN) STONES. 



