RAW QUARTZ, ITS DEFECTS AND INSPECTION 355 



the only difference being that here the crystal is viewed slightly off the optic 

 axis. Comparing the two views it is noted that here there are many more, 

 and narrower, thickness-contours.^ As the crystal is further rotated away 

 from optic-axis-viewing, the contours multiply until they are no longer 

 visible. Thus, it is by so positioning the stone as to obtain the fewest and 

 widest thickness contours that the optic axis direction is determined, the 

 axis then being jiarallel to the line of sight from eye to stone. It might be 

 parenthetically noted that the handedness of raw stones may be determined 

 in the inspect oscope, if the polarizing fdter on the viewing side be rotatably 

 mounted, by observing the contour contraction or expansion as the filter is 

 rotated (for stones progressively thicker from outer boundary inward, the 

 handedness rule is opposite to that for the conoscope). 



A further effect of tilting the stone away from optic-axis-viewing is to 

 enchance the toothed, twinning-patterns. Certain of these patterns are 

 enhanced by tilting one way, others by tilting differently (note that regions 

 .1, C and (/' are much clearer here than in Fig. 4.8). Also, since the thick- 

 ness-contours move about and the toothed patterns remain fixed, motion 

 of the stone is an aid to location of twinning (except in the rare cases of 

 large sized twins, where this doe; not hold). Note that optical twins usually 

 extend inward from the origina natural faces. The twin G which appears 

 to be internal, actually extends inward from a cap face. 



Figures 4.10 and 4.11 show projection illumination views of typical, 

 parallel BUBBLE-PHANTOMS (in stone 7, Fig. 4.5). The light from the 

 left converges into the stone, focuses about centrally, diverges and passes 

 out of the stone at B. Due to an internal fracture in the right end the light 

 is also reflected upward at C. The light beam is visible in the fluid but not 

 in the stone, because a slightly contaminated fluid scatters far more light 

 than does quartz. 



In Fig. 4.10 the stone is held so that the phantom planes A-A are viewed 

 edge on (the only way finer textured planes are visible), while in Fig. 4.11 the 

 planes are viewed at a slight angle, to show area of the planes. These planes 

 have a texture of distinguishable bubbles. The planes are long, about an 

 inch wide (with rectangular boundaries at their left end) and are parallel 

 to a possible natural face (no acutal faces present on this broken stone). 

 Such bubble phantoms are probably not permissible in any finished piezo- 

 electric plate. 



Figure 4.12 shows bubbles, cracks, veils, and phantoms (in stone 9, Fig. 4.5) 

 and pairs of angularly joined phantom planes, B-B, parallel to the natural 

 faces -4-.1 ; each pair forms two faces of interior phantom crystals. The tex- 

 ture of the phantom planes near .I-.l changes along their length from bubbly 

 at the left to bluish at the right. A dense curved blue veil is seen at C-C, 



^ Actually the thickness contours are not now as closely related to the thickness as before 

 (due to the birefringence effects being added to the rotatory effects; see Chapter II). 



