False-coloured Scanning Electron Micrograph (SEM) of microscopic crystals of calcium carbonate. Calcium carbonate has many polymorphs (different shaped) crystal forms. A multitude of hexagonal vaterite plates can be seen in this image. Calcium carbonate occurs naturally in many rocks, such as limestone and other chalky rocks and is relatively inexpensive to extract. It has a many different industrial uses and is the most common active ingredient in antacid tablets. Image width is 82 microns.