Process of producing diffraction gratings on the surface of articles

A process of producing an article having a surface provided with an optical diffraction grating such that said surface generates visible colors when illuminated with diffuse white light, which process comprises providing a starting material, and deforming said starting material to produce said article having said surface, wherein, during said deforming step, a diffraction relief pattern is impressed on said surface of the article by a tool provided with a bearing surface having a corresponding surface relief pattern of parallel lines that contacts the surface of the article under pressure, and while the diffraction relief pattern is being impressed on the surface of the article, the surface is caused to slide in a direction of movement relative to the bearing surface of the tool, and the tool is oriented such that the lines of the relief pattern on the bearing surface of the tool extend parallel to the direction of sliding of the article as the diffraction relief pattern is impressed on the surface of the article.