Calcium, with atomic number 20, is the main mineral element associated with bones and teeth.
Most of the body's calcium is stored in the skeleton, where calcium ions combine with phosphate and hydroxide in crystals of hydroxyapatite. These mineral crystals reinforce a flexible collagen framework, giving bones strength while helping teeth withstand chewing and abrasion. Calcium is therefore essential to the structure of bones and teeth, although those tissues also contain oxygen, phosphorus, hydrogen, carbon, and organic proteins.
Calcium has important roles beyond the skeleton. Dissolved calcium ions help muscles contract, nerves transmit signals, blood clot, and cells communicate. The body carefully regulates calcium levels in the blood, drawing on bone stores when necessary. Vitamin D supports calcium absorption from the intestine.
Pure calcium is a reactive, silvery alkaline earth metal and is not found freely in nature. It commonly occurs in compounds such as calcium carbonate, the principal mineral in limestone and marble. English speakers sometimes confuse calcium with phosphorus because both are major skeletal minerals. Phosphorus is indeed crucial to hydroxyapatite, but the question's canonical element answer is calcium, identified by the symbol Ca and atomic number 20.