Explanation of visual effect in smoky quartz

I have a smoky quartz cabochon with an interesting visual effect. When look at it at a certain angle range then a set of parallel grey lines is visible. When look at it at another angle it is completely clear. Attaching few photos.

Could you guys explain the nature of this effect?

The lines appear due to oscillatory zoning as sometimes seen in zircon. As the source crystal grows it experiences episodes of slow growth where impurities deposit. In a crystal these can be seen to parallel the crystal habit but when a gem is faceted these lines can be orientated to add interest to the gem. In some minerals like andalusite the impurities may form a cross as in chiastolite. Another source of such lines can be impurity zones not seen in colorless quartz but made visible by natural radiation that gives smoky quartz its natural and background body color.


Thank you, Ivan