Hydroxyapatite was synthesised by diffusion of calcium ions throughout an alkaline silica gel matrix. Pure hydroxyapatite was obtained when the initial pH value of the gel was set in the range between 10 and 12. The crystals were analysed by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy and infrared spectroscopy. After drying, a solid composite material is obtained, formed by a nanometer-scale hydroxyapatite crystalline phase embedded into an amorphous silica matrix.