Mystic quartz is similar to mystic topaz. What's the difference? Basically both are white (colorless) stones to begin with, and then the Azotic treatment is applied to give it the mystic color.
The treatment was originally developed for topaz stones. As demand grew, the scientists at Azotic have figured out how to apply their treatment to quartz as well. Why is that good? 3 reasons: (1) Quartz and topaz share similar optical properties, so you can't tell the difference visually. (2) Quartz is readily available in larger sizes. (3) Quartz has a lower specific gravity than topaz -- therefore, with two stones of the same exact size, the quartz will weigh less by about 30%, yielding tremendous cost savings.
The only difference between a mystic quartz and topaz, is the underlying gemstone that was treated. Since they share the same optical properties, you can't tell the difference. They are both treated by the same company, Azotic. We carry mystic quartz in larger sizes (usually 15mm and up) -- and it costs about 30% less than topaz, since it weighs about 30% less.