False-coloured Scanning Electron Micrograph (SEM) of microscopic crystals of calcium carbonate. Calcium carbonate has many polymorphs (different shaped) crystal forms. The spherical crystals are termed 'vaterite'. These spherical forms are made up of many hexagonal vaterite plates, which are arranged in 'rosettes' 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 multitude of different industrial uses and is the most common active ingredient in antacid tablets. Image width is 69 microns.