Calcium ions are present in rocks, bones, shells, biominerals, geological deposits, ocean sediments, and many other important materials. Calcium ions also play major roles in the retention of carbon dioxide in natural waters, water hardness, signal transduction and tissue generation. As one of the alkaline earth metals, the calcium atom has two valence electrons according to the octet rule. Up to now, the only known valence state of calcium ions under ambient conditions is +2, and the corresponding crystals with calcium ions are insulating.