Abstract: The invention provides a synthetic bone precursor material comprising amorphous calcium phosphate and a constituent that inhibits the crystallisation of calcium hydroxyapatite to amorphous calcium phosphate. The synthetic bone precursor may comprise a material of formula CaxMgyPO4 wherein Mg acts as an inhibitor for crystallisation. Other crystallisation inhibitors such as pyrophosphate may be present. The inhibitor is capable of dissolving in physiological saline. Loss of inhibitor from the precursor allows crystallisation of hydroxyapatite. The invention relates to use of a synthetic bone precursor to form bone in vivo, by a transformation of the amorphous precursor material to crystalline hydroxyapatite bone material by the leaching action on inhibitory ions body fluids. it is envisaged that the material will be useful in facilitating bone repair, inducing bone formation or assisting in the attachment of prostheses.