TOirjlMALINE 



363 



TOURMALINE. 



The name tourmaline, like garnet, is given to a group of isomorphous substances, the 

 chemical composition of which is constant and definite in its general type but variable with 

 regard to the elements which enter into it. These substances agree very closely in their 

 crystalline form, but, owing to differences in chemical composition, differ somewhat in other 

 physical characters, and it is these differences which serve to distinguish one tourmaline from 

 another. To several varieties, distinguished from each other by differences in specific 

 gravity, colour, transparency, and so on, mineralogists have given special names; and of 

 these varieties, those which are sufficiently transparent and pleasing in colour find an 

 extensive application as gems. To the ordinary jeweller the name tourmaline and the 

 several variety names recognised by mineralogists are alike practically unknown. The 

 gem-varieties of tourmaline are distinguished by jewellers solely by their colour, and are 

 referred to by the names of better-known gems to which a qualifying prefix is added. 



The physical characters of tourmaline depend more or less directly on the chemical 

 composition ; it is therefore advisable to consider this first. In order to give an idea of the 

 chemical composition of different tourmalines a table of analyses of differently coloured 

 specimens is given below. These analyses refer to : I., colourless tourmaline from De Kalb, 

 St. Lawrence Co., New York ; II., pale green tourmaline from Haddam Neck on the 

 Connecticut River, U.S.A. ; III., red tourmaline from Shaitanka in the Urals ; IV., brown 

 tourmaline from Dobrowa, near Unterdrauburg on the Drau in Carinthia; V., dark blue 

 tourmaline from Goshen, Massachusetts, U.S.A. ; VI., black tourmaline from Unity, New 

 Hampshire, U.S.A. 



