If you absolutely want to save it as *.mat files. create a sphere or whatever, apply the material (wood, tree bark...) and then save it as vob (right-click the material window, select save material). Remember you must keep the bitmap somewhere, otherwise Vue cannot open the mat file. I think the best is you just keep the bitmaps in your bitmap folder or in underfolders calle "Wood", "Barks", Grasses et.... But in this case you will NOT see it in the material preview window. Only mat files shows up there.