This is the Result:
You should take a picture with a lot of background. Then you have to create a selection (rectangle or ellipse) around the object you wish to highlight. The final decision which type of selection you will use has to be made in the menu.
Start the plugin Darken Frames Filters/Gimp-Atelier/Darken Frames. Following menu disappears
The settings for bump map and Gaussian blur are identical with original tools, so I will not describe it here.
Is Frame Bump Mapped set to No, then the values in green and blue frame are not required.
If you are working wih diamond selection, it's recommended to center minimum one axis of the selection
Now you could prepare the top layer with bump map, light or other effects and you will have a final result.
And here is the version with inverse Bumpmap:
Have a lot of fun