There are no less than three minerals or phases of CaCO 3 . Aragonite is orthorhombic , vaterite is hexagonal and calcite is trigonal . Aragonite is a common mineral, but is vastly out distanced by calcite which is the more stable mineral at most temperatures and pressures and in most environments. Vaterite on the other hand is extremely scarce and rarely seen. Aragonite will over time convert to calcite and calcite pseudomorphs after aragonite are not uncommon.
Calcite is truly one of the best collection type minerals.
There are lots of interesting forms and varieties as well as colorful and beautiful specimens to collect. It is generally easy to identify using its rhomohedral cleavage, reaction to acid and double refraction and makes for a great classroom example of these properties. If it is not the significant mineral on a specimen, it might be an accessory to other wonderful minerals and only enhancing their attractiveness. With its many different forms, environments, associations and colors, a collector could never have all possible combinations of calcite covered. Color is extremely variable but generally white or colorless or with light shades of yellow, orange, blue, pink, red, brown, green, black and gray.