150 



University of California Publications. 



[Geology 



beautiful gem stone that rivals the sapphire in color and excels 

 it in brilliancy. The color, however, although fairly character- 

 istic, is not an essential property, for very commonly parts of a 

 crystal are colorless, while occasionally perfectly colorless small 

 crystals are found. The color also varies in intensity in different 

 crystals or in parts of the same crystal. When pale it is a rather 

 pure blue When more intense it assumes a violet tint. In 

 addition to this variation in color in different parts of crystals, 

 there is a difference at any one point, depending on the direction 

 in which the light passes. In other words, the mineral is strongly 

 dichroic, the ordinary ray being colorless, the extraordinary, 

 blue. A section cut parallel to the basal plane is practically 

 colorless, while sections parallel to the principal axis show the 

 deepest color. To get the finest effect, therefore, gems should 

 lie cut with the table parallel to the principal axis, and this is in 

 contrast to the sapphire, which shows its color best when cut 

 perpendicularly thereto. If such a section, cut so as to give the 

 strongest color effects, be examined with a dichroscope, the con- 

 trast between the images is most striking. The image of the 

 extraordinary ray being freed from the colorless image of the 

 ordinary ray, presents a remarkable intensity of color, very much 

 deeper, of course, than can be seen by looking at the mineral in 

 any direction with the unaided eye. In the lighter parts this 

 color of the extraordinary ray is a slightly greenish blue inclin- 

 ing to indigo as it becomes darker, and is very similar to one of 

 the axial colors show n by some cordierites; but in the more highly 

 colored or thicker parts it is an intense purplish blue. 



Tbe color is not affected by heat up to the melting point 

 of the mineral. Fragments heated to a rather bright red and 

 maintained at that heat, just short of fusion, for five minutes 

 showed no change whatever on cooling. 



Benitoite occurs generally in individual simple crystals scat- 

 tered through the matrix and varying from a few millimeters to 

 about two centimeters across. The matrix being translucent 

 white, the blue transparent crystals stand out prominently and 

 often show crystal faces. 



It crystallizes in the hexagonal system, trigonal division. 

 The observed forms are the basal plane, the plus and minus 



