In bone, biomineralization induced by osteoblasts is known to be initiated by small extracellular vesicles referred to as "matrix vesicles". Matrix vesicles possess many enzymes and transporters, which synthesize and incorporate Ca²⁺ and PO4⁻ into the vesicles. Calcification initiates when crystalline calcium phosphates are nucleated inside these matrix vesicles, and calcium phosphates, i.e., hydroxyapatite crystals, grow and eventually break through the membrane to get out of the matrix vesicles. Exposed calcium phosphates featuring "ribbon-like" appearance assemble radially, forming spherical mineralized structure, referred to as "mineralized nodule" or "calcifying globule". This process is called "matrix vesicle mineralization". Thereafter, the mineralized nodules make contacts with surrounding collagen fibrils, extending mineralization along with their longitudinal axis from the contact points of collagen fibrils - collagen mineralization. Matrix vesicle mineralization and subsequent collagen mineralization are classified as primary mineralization associated with osteoblastic bone formation. After primary mineralization, secondary mineralization takes place, gradually increasing mineral density of bone matrix. This review will introduce the microscopic findings on matrix vesicle mineralization and subsequent collagen mineralization.