Calcium is a metallic chemical element (atomic number 20) and is the primary component of bones and teeth (in the form of carbonated hydroxypatite). It is also important in many chemical functions throughout the body, particularly the transmission of nerve impulses and the function of muscles. It is, by a large margin, the most common metal found in the body and the fifth most common element in the body (after oxygen, carbon, hydrogen and nitrogen). The body can actually meet its other needs for calcium by dissolving it out of bones. Calcium is found in many foods, primarily milk, leafy vegetables, and canned fish. Vitamin D is required for the body to properly metabolize calcium.
In its pure form, calcium is a silvery-grey metal that reacts quickly with oxygen. As such, it is usually found as a part of compounds, such as chalk and limestone.