354 



MINERALOGY 



sible in the type in which quartz crystallizes. When the rhom- 

 bohedrons are unequally developed, the plus rhombohedron r is 

 usually the larger and has a high luster, in fact is more perfect 

 than the minus rhombohedron z, which may be dull, and faces 

 occurring in the same zone with it and the prism face beneath it are 

 also apt to be dull, while the faces appearing under r will be bright. 

 Since the right and left trigonal trapezohedrons are rare forms or 

 restricted to noted localities, it is not always possible to determine 





FIG. 423. Right- (a) and Left- (6) handed Smoky Quartz Crystals from St. Goth- 



ard, Switzerland. 



whether any given specimen is a right- or left-handed crystal; but 

 when the trigonal trapezohedron is present, the crystal is a right- 

 handed one when the face appears in the upper right-hand corner 

 of the prism face, or just below the right-hand corner of r, and 

 left-handed when in a similar position in the left-hand corner. 



The trigonal pyramid s is a member of the same zone as m, x, and 

 z, and will appear between z and x ; striations on s parallel to the 

 intersection of s and r are characteristic, and will determine the 

 character of the crystal when x is absent. Holding the crystal in 

 the usual position, when these striae run from northeast to south- 

 west, as directions are considered on a map, the crystal is right- 



