Solid calcium can be found in sedimentary rock in the earth's crust, and in the minerals gypsum, dolomite and calcite. Elemental calcium is not found naturally on earth because it easily reacts to oxygen and water and forms compounds.
Calcium ranks as the fifth most plentiful element in the earth's crust. It makes up about 4.2 percent of the crust.
Another form of calcium is metallic calcium. Sir Humphry Davy first isolated this form in 1808. Metallic calcium is obtained by displacing the atoms of aluminum with calcium atoms in extremely hot, low-pressure containers.
Calcium has the lightest density of all the earth's alkali metals. It is so soft that calcium can be cut with a knife, yet it can be harder than lead. Unlike magnesium, calcium is not easy to ignite; however, once combustion occurs, calcium has a brilliant, bright red luminescence.
When calcium reacts with water, it rapidly produces hydrogen gas but exudes very little heat. Calcium has a higher level of electrical resistance than aluminum or copper. Its electrical conductivity is about 40 percent of that of copper.
Calcium is the most abundant mineral found in the human body. An adult human body contains about 1 kilogram of calcium. Ninety-nine percent of this is stored as calcium phosphate in bones.