Often the appearance of an article is attributable to the nature of the surface as much as its shape.  Reflective surfaces are often depicted in design patent drawings as parallel lines, as in U.S.Patent No. D754125:

The test completes the disclosure: “The oblique shade lines in the Figures show a transparent, reflective, or shiny surface, and not surface ornamentation” (emphasis added).  Sometimes the surface is not reflective as much as the material is reflective.  This is commonly handled withing shading, and again an explanation in the text, as in U.S. Patent No. D733,397::”United States Patent: D733397 The stipple shading represents claimed reflective piping” (emphasis added).