RAW QIAKTZ, ITS DEFECTS AND IXSPECT/ON 339 



With defaced quartz (river and broken) the orientation of the crystal 

 structure cannot be determined from the surface shape. Since the stones 

 must be cut at specific orientations relative to this structure, special means 

 must be employed to determine the structure orientation. For this reason 

 many users of quartz prefer faced stones. However, defaced stones are 

 usually more free from defects than faced stones, since optical twinning is 

 commonly concentrated near the natural faces of the original stone and 

 other defects near the base. Thus by making use of special means (in- 

 spectoscope, conoscope, oriascope) defaced quartz may be cut to good 

 advantage. 



Fig. 4.1 — ()uartz may show smoky or citrine coloring throughout or only in restricted 

 regions, irregular!}- as at the left, or in PPI.\NTOM [)lanes as at the right. 



4.3 Color 



Usable quartz is transparent internally (though the exterior surface may 

 be opaquely coated), never translucent (milky). The color of the quartz 

 varies from perfectly clear through slightly smoky to fairly dark. The 

 obviously dark stones are called SMOKY QUARTZ. Smokiness may be 

 uniform throughout a piece, or varying from clear to dark, or confined to 

 plane sheets within a single piece, see Figure 4.1. Dark smoky stones are 

 not used because they cannot be inspected for defects and optic axis. With 

 stones that are used this coloration is seldom so dense that it may be detected 

 in the small finished plates with ground surfaces. 



Because the smokiness is due lo so slight a deviation from the pure quartz 

 its analysis is extremely difficult. The coloration is variously explained, as 

 due to minute traces of impurities (organic or inorganic), as due to the dis- 

 sociation of a few Si02 molecules into free silicon and oxygen, and otherwise. 



An important fact about smoky quartz is that it may be cleared of colora- 



