108 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



The matter of color is of interest. Some specimens are of one 

 color only; others are green at one extremity and red at the other; 

 some are green, then yellow, red, and finally green; others are crim- 

 son, tipped with black, or dark green passing into blue. A crystal 

 may be white at the termination, then green of varying shades, pink 

 and colorless, and in cross section dark blue or red at the center, sur- 

 rounded by concentric layers of white, pink, and green. Another 

 specimen may be red internally, passing into a lighter hue and finally 

 green, or it may be 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 defined. 



The optical structure of the tourmaline is unique. When a crystal 

 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 crystal, 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 only 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. 



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 muscovite. 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 with the lithia group. The mag- 

 nesia group — commonly brown in color — occurs chiefly in crystalline 

 magnesian limestones associated with mica, pyroxene, scapolite, etc. 



In the United States magnificent colored tourmalines have been 

 found in Maine at Auburn, Hebron, Norway, Andover, Rumford, 

 Standish, and Paris. (See pi. 10.) The famous locality at Mount 

 Mica, near Paris, was discovered in 1820, and for many years yielded 

 fine specimens of green and parti-colored tourmalines. Some crystals 



