Calcium is a hard, silvery metallic element which has an atomic number of 20 and an atomic weight of 40.078. It is classified as an alkaline earth element and burns with a yellow-red flame. It is highly reactive (pure calcium reacts explosively with water) and is therefore not found in its pure form in nature. Instead, it is typically found in minerals such as gypsum and fluorite, in limestone (as calcium carbonate) and stalactites/stalagmites, and in the bones, teeth and shells of animals. Calcium is essential in the human diet (people normally consume 600 1400 milligrams per day). The body needs it for a variety of functions, such as blood clotting, the transmission of nerve signals, muscle contraction, and proper heart function. Calcium imbalances can cause many health problems, and some chemical imbalances can cause the mineral to build up in nerve cells, poisoning and ultimately killing them.