Final answer:
Bone hardness is due to crystallized inorganic salts consisting mainly of calcium phosphate and calcium carbonate, which form hydroxyapatite, and are further strengthened by other minerals like magnesium hydroxide, fluoride, and sulfate.
Step-by-step explanation:
The hardness of bones depends on the crystallized inorganic salts that form part of their structure. These salts consist primarily of calcium phosphate and calcium carbonate which combine to create hydroxyapatite. Hydroxyapatite, as it crystallizes or calcifies on the collagen fibers, incorporates other inorganic salts like magnesium hydroxide, fluoride, and sulfate. The crystallization process of these inorganic salts on the collagen matrix gives bones their hardness and strength while maintaining flexibility to prevent brittleness.