This may depend on your method.
I just tested opening a revit file in Navisworks and all materials were mapped and visible.
Suggestions;
In Revit save a 3D view called 'Navisworks' with visibility of geometry set correctly for the information you wish to be exported (don't worry about 'Visual Style')
In Navisworks, press 'F12' for options>file readers>revit and set 'convert' to 'Navisworks View'. this will import the view you saved in Revit
Append the Project File.rvt from the home tab in Navisworks.
Make sure that the render style in Navisworks is set correctly, Viewpoint>Render Style>Mode>Full Render
This 'should' get you to the right place. If you check the 'Try and Find Missing Materials' in the navisworks file readers options, you will get a warning is any of the material bitmaps are missing.
Hope this helps.