The formation of nanocrystals after implantation of silver alone as well as together with the halogen ions Cl, Br and I into a SiO2 layer of about 100nm was studied by X-ray diffraction and transmission electron microscopy. The co-implantation of Ag and Cl or Br results in the formation of cubic AgX crystals which are stable in size under annealing. The co-implantation of Ag and I as well as single Ag implantation result in Ag crystallites, which grow under annealing. The annealing procedure causes a redistribution of the particles within the layer.