Why are most quartz crystals not colorless (clear) ?

Explained

Explanation:

Quartz is never found in pure form, most of the quartz crystal have some or other form of impurity in them. It is these impurities that impart color to the quartz crystal. Ions of different elements like magnesium, iron, aluminium are amalgamated with in the SiO₂ crystal imparting it different colors. The Piezoelectric properties of Quartz are also because of the same reason

(availability of free electron).