Calcite (calcium carbonate, chemical formula CaCO3) is the most important and widespread carbonate mineral on earth, occurring in many forms, like limestone, marble, shells, coral and other. This picture shows intergrown rhombohedral crystals of calcite tainted orange red by presence of some iron in the crystal lattice. Provenance is the Potosi mine in the Eulalia district of Mexico. Actual width of sample is 180 mm.