94 THE PROSPECTOR'S HANDBOOK. 



be distinguished from a spinel ruby or garnet without 

 dichroism, or from a pink tourmaline (rubellite), which 

 gives two colours, but somewhat differently to those of 

 ruby ; so, too, a sapphire, which gives a blue shade in one 

 square and a light shade of colour without any shn.d<- of 

 blue in the other (though sometimes in a deeply-coloured 

 stone what might be considered as a greenish blue is noticed), 

 can be distinguished from an amethyst, which affords two 

 shades of purple, or from a blue spinel (which does not show 

 any twin coloration), or from an iolite (or water sapphire), 

 in which the coloration is of its own kind. 



A tourmaline (very frequently associated with other gems, 



FIG. 52. 



When a transparent or serai-transparent stone is examined through the dichroi- 

 scope, the colour of the square A is different, or of a different shade, to that of 

 the square B when dichroism exists. 



especially in Ceylon), either the green or brown, can be 

 recognised directly (indeed, often without using the dichroi- 

 scope) by the colour of the one square being quite dark 

 compared to that of the other. 



An emerald affords two distinct shades of green (one 

 bluish) easily remembered (quite distinguishable from the 

 dichroism of a green tourfnaline) ; so a green garnet, 

 which does not show twin coloration, cannot be mistaken 

 for it. 



With the dichroiscope and two or three minerals, such as 

 the sapphire, topaz, and rock crystal, to test for hardness, 

 and a little practice the more the better and a slight 

 knowledge of the crystallization of minerals, which, though 

 frequently found water-worn, not uncommonly retain traces 

 of the original crystal edges and faces, the prospector 

 can examine his specimens with a very much easier mind 

 than he would do without them. 



