Calcium

Almost no one meets calcium as a metal; they meet it as calcium carbonate, which is limestone, chalk, marble, coral, eggshell and the scale that ruins a kettle element. Davy isolated the metal in 1808, again by electrolysis. Roasting limestone to quicklime and slaking it to make mortar is one of the oldest chemical processes in civilisation, and Portland cement is its industrial descendant — accounting for something like 7% of global CO₂ emissions, mostly from the carbonate decomposition itself rather than the fuel. In your body about 99% of the calcium is locked into bone as hydroxyapatite, but the remaining 1% in solution runs muscle contraction, clotting and cell signalling, and blood calcium is held within a few percent by hormonal control. In water treatment, calcium plus magnesium is hardness.