What is Calcium?
Chalk cliff in Cottonwood Canyon, Pueblo, Colorado
When a star is on the way to converting all of its carbon and oxygen atoms into iron, one kind of atom it makes along the way is calcium. One atom of calcium has 20 protons and 20 electrons (and 20 neutrons). Calcium is a light kind of metal, not as strong as iron, though it is harder than lead.
After the star is done changing its atoms to iron, it explodes, shooting the calcium atoms out into a new nebula. Many of the calcium atoms in our nebula got made into part of the Earth, so that calcium is the fifth most common element in the Earth's crust.
Most of the calcium on Earth is mixed with carbon to make molecules of calcium carbonate, which is what sedimentary rocks like limestone and chalk are made out of. But calcium is also one of the main atoms involved in making living creatures. Sponges and other animals make their hard structure out of calcium, and all chordates make their bones and eggshells out of calcium.