It is two low poly meshes, one inside the other. The one on the outside uses a .tga texture where the alpha map determines what is seen in the render, those parts where the alpha map is black appear transparent. The second, interior mesh is solid with no alpha map. Because the second mesh is just below the exterior mesh, it gives the appearence of relief. If you look too closely at the model, you will see that there is discontinuity between the meshes.