Chemical Formula: CaCO3
Color: Any color, but is usually white or clear
Importance: Calcite is part of many household items, including chalk, cement, paint, glass, fertilizer, and rubber. It is also used to make certain medicines.
Other Interesting Information: Is one of the most common minerals. Has perfect rhombohedral cleavage, which means it breaks into a perfect rhombus shape. Also exhibits double refraction, which means that when placed over an image, the image doubles.
My Take: Calcite is one of the more interesting minerals. Even though it is very common, it has so many special properties that make it special. All in all, it never fails to amaze.
For More Information: http://www.minerals.net/mineral/calcite.aspx