Calcite is calcium carbonate. It is usually white or colorless, but can be other colors because of impurities in it.  It may have many outward shapes, but always breaks into little pieces  shaped like rhombs (Like a box - only slanted). Calcite makes up the rock called LIMESTONE and is used to make cement, as fertilizer and as building stones.

Calcite is a common rock-forming mineral. It has many industrial uses, such as cement, and is found in many colors. It occurs in many forms of crystals, and in many types of deposits, and has a strong rhombohedral cleavage. An optically clear variety is called Iceland spar, and exhibits strong double refraction, which is when a ray of light enters the mineral, it is divided into two rays that diverge. It is too soft to be used as gem material, but many beautiful crystals have been collected the world over.


More information is available at the folowing links:

http://en.wikipedia.org/wiki/Calcite

http://www.mindat.org/min-859.html

http://www.galleries.com/minerals/carbonat/calcite/calcite.htm