CATALOGUE OF GEMS. 533 



^Teen; or it may ))0 blue or black internally, then red, and then green 

 externally. In some specimens the different colors pass imperceptibly 

 into one another; in others the line of demarcation is well detined. 



The colorless tourmalines are called Achroite; to. the black the 

 names Aphrislte and Schorl are applied; the blues, either pale, indigo, 

 or l)lackish blue, are called Ind'acVde; this also includes the kinds 

 known as Brazilian, sapphire having a transparent Berlin-blue color; 

 the red tourmalines, varying from a tine ruby red to violet red 

 {siheritt) and pale rose red or pink, are called liuheUitt'. 



The optical structure of the tourmaline is unique. When a cr\-stal 

 is viewed along the direction of its vertical axis it is less transparent 

 and of different color than when viewed across that axis. For instance, 

 a crystal viewed through the side is a transparent green, but when 

 viewed through the end of the prism it may be either opaque or yellow 

 green. 



The marked pleochroism of the colored tourmalines influences to a 

 great degree the appearance of the fashioned stone. For example, if 

 a green-colored specimen is cut so that the table is parallel with the 

 vertical axis of the crj'stal, the gem will exhibit a play and interchange 

 of colors of two shades of green; if, however, the specimen is so cut 

 that the table of the fashioned stone is perpendicular to the vertical 

 axis, the gem will appear more or less opaque and dark colored, and 

 will exhibit its transparency and green coloring onh' when viewed 

 across the girdle. Care should be taken, therefore, in fashioning the 

 tourmaline that the table is parallel with the vertical axis of the 

 crystal; further, the facets of the crown should be large and well- 

 developed in order to exhibit to the utmost the differences of color for 

 light transmitted in different directions as the gem is viewed from 

 different positions. 



In composition the tourmalines are very complex boro-silicates of 

 aluminum, magnesium, iron, or alkalies, falling under four types: 

 Lithia tourmalines, iron tourmalines, magnesia-iron tourmalines, and 

 magnesia tourmalines. 



The geological occurrence of the four types of tourmaline is of 

 interest. The lithia group — which is often beautifully colored and 

 affords the best gem material — is associated with soda and potash feld- 

 spar in pegmatite veins along with lepidolite and umscovite. The iron 

 and the magnesia-iron groups, which are commonly black or brownish 

 black, occur in granites, gneisses, schists, and also to a certain extent 

 in pegmatites along Avith the lithia group. The magnesia group — 

 commonly brown in color — occur chiefly in crystalline magnesian 

 limestones associated with mica, pyroxene, scapolite, etc. 



Tourmaline is usually found in granite, gneiss, or mica slate. It 

 occurs also in dolomite or granular limestone, and in certain contact 



