What are human bones made of?

The main component of bone is bone matrix, which is a mixture of a fibrous protein called collagen and carbonated hydroxyapatite, an inorganic compound mostly made of calcium and phosphate. The combination of fibrous collagen and crystallized calcium makes bones hard and rigid and adds tensile strength.

In children, the ends and edges of bones are mostly flexible cartilage, which is replaced by hard bone matrix as the bones reach their full size. Inside most of the larger, longer bones in the body, there is a flexible core of tissue called marrow, which is responsible for the production of both red and white blood cells.