Calcium was isolated by Humphry Davy in 1808, though its compounds are among the most familiar substances on Earth. The name comes from the Latin calx, meaning lime, the material of limestone, chalk, marble and quicklime. Calcium is a soft grey alkaline-earth metal, far too reactive to occur free, but bound up as carbonate, sulfate and phosphate it forms mountains, shells, coral and the bones and teeth of animals. The fifth most abundant element in the crust, calcium is vital to living things, hardening bone and shell and triggering muscle contraction and the clotting of blood, while its compounds lime and gypsum remain foundations of building and agriculture.
Vast formations of limestone, including the White Cliffs of Dover, are compressed calcium carbonate laid down over millions of years from the microscopic shells of marine organisms, so that entire landscapes are, in effect, fossilised calcium. In the body the element reaches its hardest form in tooth enamel, built from a calcium phosphate mineral called hydroxyapatite, the toughest substance the human body produces. Gypsum, another common calcium mineral, has been carved as alabaster and burned to make plaster of Paris, used for centuries in sculpture, wall finishing and, in modern medicine, for setting broken bones.