the faceted diamond is one solution, essentially analogous to planar mirror surfaces which never present a useful angle of reflection.
the aplanar curve is another solution, analogous to a mirror surface which spreads the angle of reflection to a maximally diffuse (ie: weak) range, producing a very small and vague signal return.
the copper mesh and carbon-fiber materials both planes use to absorb some RF is yet a third solution.
the grids covering the intakes on the F117 is a fourth solution - the grid size allows RF to enter, but not to reflect.
don't assume so much.
see post 46