What hardens bone tissue in the human body?

Bone tissue hardens primarily due to the presence of calcium salts, which are deposited within the organic matrix of the bone. The most significant of these salts are hydroxyapatite crystals, formed primarily from calcium phosphate. When calcium salts mineralize, they provide strength and rigidity to the bone structure, allowing it to withstand the physical stresses it encounters throughout life. This mineralization process is critical for the proper formation and maintenance of bones, contributing to their strength and overall function. While other factors like collagen fibers provide the organic framework and flexibility, it is the calcium salts that are pivotal in the hardening process.