RAW QUARTZ, ITS DEFECTS AND LWSPECTION 353 



if too far separated). This ring pattern is due to conical illumination (or 

 more correctly to conical viewing; here the illumination of the stone is 

 diffuse). This simply illustrates the basic principle of the conoscope. The 

 principle is further illustrated by tilting the section out of its present posi- 

 tion, which causes the ring system to move in the direction of tilting. The 

 theory of these effects is given in Chapter II. 



Figure 4.8 is a polarized light view of a pyramidal cap of quartz with a 

 fractured back surface (stone 3, Fig. 4.5). Here the stone is viewed along 

 the optic axis (the six natural cap faces making equal angles with the line 

 of sight). The continuous dark bands are thickness-contours, and again 



Fig. 4.7 — A parallel faced, basal section of quartz viewed along the optic axis in polar 

 ized light. Note the vertical thickness-contour at B. The conoscope ring-pattern C 

 may be obtained by introducing a lens. 



represent regions of equal thickness in the optic axis direction. Had the 

 fractured surface been flat instead of broken these contours would have 

 been hexagonal and parallel to the hexagonal edges of the cap. The toothed- 

 patterns at -4 to G are due to optical twinning (thin layers of the quartz 

 whose handedness is opposite to that of the main stone). Although the 

 exact shape and location are not determinable, the approximate location 

 and extent are observed by tilting the crystal while viewing. The contour 

 and pattern changes resulting from angularly moving the crystal (away from 

 the position of viewing directly along the optic axis) are shown by Fig. 4.9, 

 which should be compared with this figure. 



Figure 4.9 is a polarized light view of the same stone as shown in Fig. 8, 



