Not all calcite crystals perfect; synthesis methods can alter internal structure, affect chemical reactivity

When looking at calcite under a microscope, a scientist would immediately recognize the crystalline form of calcium carbonate by its rhombohedral appearance. That is, calcite is shaped like a distorted cube. One of Earth's most abundant minerals, calcite is a major component of limestone and marble. It is also the most stable of the three common, naturally occurring crystal forms of calcium carbonate; the other two forms are aragonite and vaterite.