BONE MATRIX (OSTEOID) Inorganic matter Calcium, phosphorus, bicarbonate, magnesium, etc. Calcium and phosphorus form hydroxyapatite crystals Organic matter Type I collagen (90%) Ground substance of proteoglycan aggregates and multiadhesive glycoproteins (e.g., osteonectin) Mixture of minerals with collagen fibers (i.e., hydroxyapatite crystals are deposited in the gap regions along the collagen fibers) produces the hardness and resistance of bone.