Programmed iodization of silver foils at ambient temperature and pressure leads to the formation of gamma-AgI films with zinc-blende structure. Gravimetric experiments show that the AgI film formation follows a parabolic time dependence. X-ray diffraction monitoring of the film formation on unannealed, annealed and etched silver foils reveals that the absence of intrinsic stacking faults is a necessary condition for the formation of gamma-AgI thin films and that the presence of such faults invariably results in the formation of beta-AgI thin films. Optical absorption studies indicate eventual saturation of the direct optical band gap with increase in iodination time, which corroborates Xray diffraction and gravimetry results.