Calcium is a chemical element, number 20 on the periodic table. (TOS: "The Omega Glory")
During a scan of Aldean DNA in 2364, Doctor Beverly Crusher was able to confirm that inefficiencies in calcium resorption was not responsible for the Aldeans' sterility. (TNG: "When The Bough Breaks")
Calcium left trace patterns by which a Doctor could establish a time period in which bones were broken. Beverly Crusher used this method to learn that Jono had broken his arm and fractured two of his ribs in 2360. (TNG: "Suddenly Human")
A sudden decrease of serum calcium levels increased muscle spasm. This was a side effect of radiation poisoning. The pain caused by this could be counteracted by asinolyathin. Chief Miles O'Brien suffered from this when he had his visions in 2371. (DS9: "Visionary")