532 



REPORT OF NATIONAL MUSEUM, 1900. 



There arc several distinct minerals which are commonly called topaz, 

 the 3-ellow sapphire known as the " oriental topaz," the topaz proper, 

 and certain colored kinds of quartz, known as "Saxon," "Scotch," 

 "Spanish," "smoky," and "false topaz." These stones vary rather 

 wideh^ in hardness and specific gravity, which, together with the power 

 of developing frictional electricity (possessed by the true topaz) fur- 

 nishes a rcadv means for their discrimination. Thus: 



Name. 



Oriental topaz... 

 True topaz 



Scotch topaz, etc 



Hard- 

 ness. 



Specific 

 gravity. 



-1.01 

 3.53 

 2. f.5 



Topaz occurs in g-neiss or granite, associated with tourmaline, mica, 

 beryl, etc., and occasional!}' with apatite, fluorite, and cassiterite; 

 occurring also in certain talcose rocks, in mica slate, in rhyolite, and 

 in alluvial deposits and drift. 



In composition the mineral is a fluo-silicate of alumina, containing 

 in 100 parts: Silicon, 15.5; aluminum, 30.2; oxygen, 36.8; fluorine, 

 17.5. 



TOURMALINE. 



ACHROITE — APHRIZITE — INDICULITK — KUBELLITE. 



The tourmaline crystallizes in the rhombohedral division of the 

 hexagonal system. The crj^stals are commonly prismatic in habit, 

 and often slender to aeicular. The prismatic faces are strongly 

 striated longitudinally, giving a rounded, barrel-shaped, or triangular 

 appearance to the crj^stal. The crystals are sometimes isolated, but 

 occur more commonly in parallel and radiating groups. The mineral 

 also occurs massive, compact, or in parallel to divergent, columnar 

 shapes. Hardness, 7 to 7.5; specific gravity, 2.9tl: to 3.3; luster, vit- 

 reous; transparent to opaque, and unlike in transparenc}" across the 

 prism and in the line of the axis; becomes electric by friction. Color, 

 black, brown, blue, green, red, and white; some specimens red inter- 

 nally, and green externally; others red at one extremity and green, blue, 

 or black with intermediate shades at the other; dichroism marked. 



The question 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 crimson, 

 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 



