Calcium, you know what that is, yes? Bones are made of it, right, and teeth, and limestone rocks and stuff?
You’ve probably never seen calcium in your life. And certainly never handled it. For calcium is a bright, silvery metal, and all the white rocky material, including your bones and teeth, are made of calcium rust.
A piece of iron, or particularly steel, if left lying around outside for a week or so, will form a red-brown powder on its surface; rust, of course. If left long enough, the entire piece will be converted to rust by the action of air and moisture. Some soil consists mainly of rust; those rich, red soils, mined as ore, are actually iron rust, just as all that limestone in the land, like the White Cliffs of Dover, is mostly calcium rust.
The bodies of all creatures use calcium rust to make bones and teeth and shells, and seek it out in the environment for food, as we do.
Strontium is a very similar metal to calcium, which is a worry, because when fallout from a a nuclear disaster contains radioactive strontium 90 ‘rust’, our bodies will absorb it as if it were useful for making bone and tooth, causing sickness and death.