Calcium is a reactive silvery metal that is the fifth most abundant element in the earth's crust, at about three percent by weight, and the third most abundant metal, after aluminum and iron. It occurs only in compounds, the most common of which is calcium carbonate, the main constituent of the fossilized remnants of early sea life and limestone.
Calcium is an essential element for both animals and plants. It has several important functions including as a structural element (especially cell membranes, bones and teeth), as a signal transducer, and as an enzyme cofactor. In higher animals it is the most abundant inorganic element, and in humans it is the fifth most abundant element and the most abundant metal at two percent. Major sources of calcium in the human diet include milk and milk products, beans, fish, and green leafy vegetables.