For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
In the same sense that we contain a massive volume of gas, because there is a lot of hydrogen in our bodies. Yes, hydrogen is a gas, and yes, there is a lot of it on our body. But it’s bound, so it doesn’t count.
It would be more accurate to call it stone than metal, because the calcium in our bones is also bound to other elements, which means it does not exhibit its usual metal characteristics.