You're on the right track with the mia_material_x. I probably wouldn't use a marble texture as it's making the gem appear kind of cloudy. If anything, since the camera is so close, I'd maybe use a modified solid fractal texture (high amplitude, high threshold etc) in the bump channel to create tiny imperfections. If you're going for realism then there is work to be done with the lighting and environment as well. Lots of studio lighting tuts floating around.