Bone is not a single material — it is a biological composite, meaning it is built from two fundamentally different substances working together.

The first is collagen, a protein that forms long, interwoven fibres throughout the bone's structure. Collagen is tough, flexible, and resistant to being pulled apart — it gives bone tensile strength, the ability to resist snapping under a bending force. On its own, collagen is rubbery and bendy, like the cartilage in your nose and ears.

The second is calcium mineral — primarily calcium phosphate in a crystalline form called hydroxyapatite, with some calcium carbonate. This mineral is deposited throughout the collagen framework during bone development, hardening it into a rigid structure that resists compression and supports the body's weight.

Together, these two materials give bone a remarkable combination of properties: hard enough to support the body and protect organs, yet flexible enough to absorb shock without shattering. Engineers recreate this same principle in composite materials: reinforced concrete (concrete plus steel), fiberglass (glass fibres plus resin), and carbon fibre all use two materials — one rigid, one flexible — to produce something stronger than either alone.