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Calcite is the one of the most common minerals on earth. It occurs in a seemingly unlimited variety of shapes and colors. It constitutes a major portion of many of the earth’s rocks. Calcite belongs to the calcite group of minerals, a group of related carbonates that are isomorphous with one another. They are similar in many physical properties, and may partially or fully replace one another, forming a solid solution series. All members of the calcite group crystallize in the trigonal system, have perfect rhombohedral cleavage, and exhibit strong double refraction in transparent rhombohedrons.

Calcite and Aragonite are polymorphous to each other. Although Calcite and Aragonite contain the same chemical composition, they differ in crystal structure. Calcite forms trigonal crystals, whereas Aragonite forms orthorhombic crystals. Sometimes the crystals of Calcite and Aragonite are too small to be detected, and it is only possible to distinguish these two minerals by complex scientific optical tests. Since the true identity of microcrystalline forms of Calcite or Aragonite may not known, they may be mislabeled as the wrong mineral.

A microcrystalline type of Calcite in globular form is common in certain regions. This Calcite forms from precipitating calcium-rich water inside caverns or on limestone cliffs. It exists in the form of stalagmites, stalactites, flowstone, and strange globular growths. These growths constantly accumulate, forming layers. They are frequently impure, trapping in organic matter such as leaves, twigs, and moss as they accumulate. Because of their impure status, they are classified by some as rocks. These calcareous growths have designated names based on their shape, habit, or formation. Most of these growths are Calcite, but some are crystallized as Aragonite. The environment of formation, however, can be a key guide to whether the mineral crystallized as Calcite or Aragonite. Aragonite will generally develop only at hot springs, whereas most other calcareous growths will be Calcite.

Calcite may form as an undesirable coating on top of another mineral. The calcite can be easily burned off by soaking it in acid, which will cause it to effervesce and eventually dissolve, leaving the mineral below exposed.