It's Elemental

Although calcium is the fifth most abundant element in the earth's crust, it is never found free in nature since it easily forms compounds by reacting with oxygen and water. Metallic calcium was first isolated by Sir Humphry Davy in 1808 through the electrolysis of a mixture of lime (CaO) and mercuric oxide (HgO). Today, metallic calcium is obtained by displacing calcium atoms in lime with atoms of aluminum in hot, low-pressure containers. About 4.2% of the earth's crust is composed of calcium.

Due to its high reactivity with common materials, there is very little demand for metallic calcium. It is used in some chemical processes to refine thorium, uranium and zirconium. Calcium is also used to remove oxygen, sulfur and carbon from certain alloys. Calcium can be alloyed with aluminum, beryllium, copper, lead and magnesium. Calcium is also used in vacuum tubes as a getter, a material that combines with and removes trace gases from vacuum tubes.

Calcium carbonate (CaCO₃) is one of the common compounds of calcium. It is heated to form quicklime (CaO) which is then added to water (H₂O). This forms another material known as slaked lime (Ca(OH)₂) which is an inexpensive base material used throughout the chemical industry. Chalk, marble and limestone are all forms of calcium carbonate. Calcium carbonate is used to make white paint, cleaning powder, toothpaste and stomach antacids, among other things. Other common compounds of calcium include: calcium sulfate (CaSO₄), also known as gypsum, which is used to make dry wall and plaster of Paris, calcium nitrate (Ca(NO₃)₂), a naturally occurring fertilizer and calcium phosphate (Ca₃(PO₄)₂), the main material found in bones and teeth.