Chemical Formula: SiO2
Color: Can be any color, but is usually white or clear
Importance: Made into many different gemstones for jewelry. Makes up sand, which is made into glass. Part of many electronics, including radios and watches. Also used to make silicon semiconductors.
Other Interesting Information: Although it looks like it has numerous cleavages, quartz does not cleave at all. Instead, it fractures. Quartz is also the most common mineral on earth and makes up parts of most rocks.
My Take: Personally, I think crystal quartz is one of the coolest looking minerals. Even though it is quite common, it is fascinating that it naturally forms in the shape that it does.
For More Information: http://www.minerals.net/mineral/quartz.aspx