Quartz is a clear, irregular crystalline structure with a milky luster. It is a common mineral, important for the creations of such things as oscillators and watches. Quartz is also considered a semi-precious gemstone.
The appearance of quartz depends on the variety. Pure quartz is a clear crystal. Citrine quartz can be clear yellow to opaque brown crystal. Rose quartz is a milky light pink to deep red crystal. Amethyst crystals are shades of purple, usually opaque. Smoky quartz is a gray crystal that can be clear or opaque. Milk quartz is a cloudy, white crystal, and the most common form of quartz. You can find more information here: http://en.wikipedia.org/wiki/Quartz#Varieties_.28according_to_color.29