I think it's probably on a flat surface. One way people get this kind of shot is to arrange a glass 'table top' somewhat above a white (or other color) surface. You can light the background independently of the foreground and there will be no shadows as there could be if you laid the necklace directly on a surface. If the spacing between the glass and the background is large enough the any texture in the background will not be in focus and may not be visible.