Not Medical Advice: It's the 'hydroxyapatite', a calcium phosphate compound makes bones hard and mineralized.
The hardness and rigidity of bone is due to the presence of mineral salt in the osteoid matrix, which is a crystalline complex of calcium and phosphate (hydroxyapatite). Calcified bone contains about 25% organic matrix (2-5% of which are cells), 5% water and 70% inorganic mineral (hydroxyapatite).
We are born with about 300 soft bones. During childhood and adolescence, the cartilage grows and is slowly replaced by hard bone. Some of these bones later fuse together, so that the adult skeleton has 206 bones.
Learn more about bones including its function at Iofbonehealth.org.
As you age, your bones become thinner and lose their density. Over time, you become more prone to injury.
Fortunately, you can take steps to halt the “thinning” of your bones, called osteopenia, and prevent osteoporosis. Check out these 7 tips to have healthy bones shared by Clevelandclinic.org.