How does calcium phosphate make bone hard?

Bone hardness is primarily the result of bone mineralization, a process where calcium and phosphate are deposited into the collagen matrix of the bone in the form of hydroxyapatite crystals.12

These crystals are deposited into the organic matrix by osteoblasts, which release matrix vesicles that act as nucleation sites for crystal formation. As these crystals grow and propagate throughout the collagenous framework, they provide the rigidity and compressive strength characteristic of bone tissue.23

The hardness of bone is fundamentally a two-phase composite effect: while the mineral crystals provide rigidity and compressive strength, they are brittle on their own. By being intimately associated with the elastic collagen fibers, the mineral component is prevented from propagating stress failures, resulting in a material that is both strong and capable of withstanding impact. Maintaining the optimal ratio of these minerals to the organic matrix is essential for skeletal health, as deviations can lead to increased brittleness or excessive flexibility.3