Abstract: The present invention is a method of marking a metal device with an identification symbol or a serial number which is not detectable by a visual examination. The method of marking includes the steps of locally heating a specified area of the metal so that its microstructure is altered. For example, a laser, the intensity of which has been adjusted so as to maintain a designated area at a temperature within an appropriate temperature range for the particular metal, may be used to trace out either an identification symbol or serial number. The method is used in combination with the step of etching away the surface area of the metal device which includes both unaltered microstructure and altered microstructure with a selected chemical etching solution that will etch away the altered microstructure at a different rate than the unaltered microstructure thereby exposing the mark.