Bone is roughly 60% mineral, 30% protein, and 10% water by weight. That mineral-protein-water recipe is what gives your skeleton its unusual combination of hardness and flexibility. Pure mineral would shatter like chalk; pure protein would bend like rubber. The mix of both is what makes bone remarkably tough.
The Three Main Components by Weight
By weight, the largest share of bone tissue is inorganic mineral, accounting for about 60%. The organic portion, almost entirely protein, makes up around 30%. The remaining 10% is water. Those proportions shift when you measure by volume instead of weight, because mineral is dense and compact while protein fibers take up more space. By volume, the split is closer to 40% mineral, 35% protein, and 25% water.
The Mineral: Calcium Phosphate Crystals
The mineral component of bone is a crystalline form of calcium phosphate called hydroxyapatite. Its chemical formula is Ca₅(PO₄)₃OH, meaning each unit contains five calcium atoms, three phosphate groups, and one hydroxyl group. These tiny crystals pack tightly along and between the protein fibers, giving bone its rigidity and compressive strength. Hydroxyapatite is also the body’s main calcium and phosphorus reservoir: when your blood calcium drops, your body pulls it from these crystals to keep muscles, nerves, and the heart functioning.
The crystals in living bone aren’t pure hydroxyapatite, though. They’re better described as “carbonated hydroxyapatite” because carbonate ions substitute into the crystal structure. Other elements slip in too. Magnesium, sodium, and zinc occupy spots in the crystal lattice where calcium would normally sit. Even unwanted elements like lead or gadolinium can accumulate in bone the same way, which is why bone sometimes acts as a long-term storage site for environmental toxins.
The Protein: Mostly Collagen
Of the organic 30%, about 90% is type I collagen. Collagen molecules are built from three intertwined chains of amino acids that twist into a rope-like triple helix. These helices bundle into fibrils, and the fibrils assemble into larger fibers, creating a scaffold that gives bone its flexibility and tensile strength. Think of collagen as the rebar inside reinforced concrete: the mineral resists compression, while the collagen resists pulling and bending forces.
The remaining 10% of the organic matrix is a collection of smaller, non-collagenous proteins. Three of the most important are osteocalcin, osteopontin, and bone sialoprotein. Osteocalcin helps stabilize the hydroxyapatite crystals and binds calcium ions. Osteopontin and bone sialoprotein help cells attach to the bone surface and also bind calcium, playing roles in how bone is built and remodeled over time. These proteins may be a small fraction of bone’s total weight, but they act as chemical signals that regulate how quickly bone forms and how mineral crystals grow.
Water’s Role in Bone
The 10% water figure refers to bone tissue measured in a lab setting. In a living person, bone contains closer to 31% water, according to data compiled by the U.S. Geological Survey. Some of that water is “bound water,” locked into the collagen matrix and the surface of mineral crystals, where it contributes to bone’s ability to absorb energy without cracking. The rest is “free water” that flows through tiny channels in bone, carrying nutrients to bone cells and transmitting mechanical signals that help the skeleton sense and respond to loading.
Compact vs. Spongy Bone
The chemical ingredients are the same in both types of bone tissue, but the proportions differ slightly. Compact (cortical) bone, the dense outer shell of every bone, has a higher mineral density, averaging around 1,040 mg of hydroxyapatite per cubic centimeter. Spongy (trabecular) bone, the honeycomb-like interior found at the ends of long bones and inside vertebrae, averages about 966 mg per cubic centimeter. That roughly 7% difference in mineral concentration means compact bone is stiffer, while spongy bone is more flexible and better at absorbing impact.
Higher mineral density also correlates with greater stiffness but lower toughness. In practical terms, heavily mineralized bone resists bending well but is more prone to sudden fracture if pushed past its limit, while less mineralized bone can deform slightly before breaking.
How Bone Chemistry Changes With Age
Bone is not a static material. Specialized cells continuously break down old bone and replace it with new tissue, a process called remodeling. As you age, the chemistry of bone shifts in several ways that collectively make it more brittle.
Bone mineral density in the hip and femoral neck declines with age, meaning there is less hydroxyapatite per unit of bone. At the same time, collagen fibers accumulate chemical modifications called advanced glycation end-products (AGEs), which act like unwanted glue between collagen molecules. These cross-links stiffen the collagen scaffold, reducing its ability to flex under stress. The result is bone that is simultaneously less mineralized overall and stiffer at the fiber level, a combination that increases fracture risk.
Bound water content also decreases with aging, further reducing bone’s capacity to absorb energy. Non-collagenous proteins lose some of their chemical activity as well: osteopontin, for example, shows about a 30% drop in phosphorylation (a chemical tag that activates the protein) by the ninth decade of life. Microcracks, tiny fractures at the microscopic level, accumulate faster than the body can repair them. All of these chemical changes contribute to the increased fragility of older bones, independent of overall bone density measured on a standard scan.
Why the Chemistry Matters
Understanding what bone is made of explains why nutrition and lifestyle affect skeletal health so directly. Calcium and phosphorus are needed to build hydroxyapatite. Vitamin C is essential for collagen synthesis. Protein intake supplies the amino acids that form the collagen scaffold. Weight-bearing exercise stimulates bone cells to deposit new mineral and maintain water flow through the tissue. Each of these inputs targets a specific chemical component of bone, which is why no single supplement or habit is enough on its own to keep the skeleton strong.