Calcium a chemical element vital to living things. In its pure form it is a light, whitish metal; but it is seldom thus seen because it reacts violently with water to form lime (calcium hydroxide).
The bones and teeth of animals, including people, are largely composed of calcium sulphate. Calcium ions also play an important role in the working of the nerves and muscles, and in the clotting of blood. Quite a lot of calcium is required in the diet. An adult needs about 500 mg a day; growing children need slightly more, and pregnant and lactating women about 1,200 mg.