Thank you for your response. You don't have to go to the opposite extreme with respect to appropriate citation / attribution of images -- just don't lie or suggest that you did something that you really didn't do.
With respect to your original question, as suggested above, by far, the best way to get rid of this problem is not in post processing, but with good basic photographic technique.
Your use of a light cube is a nice way to provide a general level of diffuse ambient illumination in product photography, but is the opposite of what one needs to bring out sparkle or "fire" in diamonds. Go to almost any high end jewelry store and notice the lighting they use to show their merchandise. In most cases, you will see that they have a large number of very small, very intense lights in the ceiling. These lights will be reflected towards your eye by the facets of the diamond and will give a diamond a beautiful sparkling appearance. For photography, having only this type of lighting will make the contrast level much too high, so a combination of a multitude of point sources PLUS a moderate level of highly diffuse ambient light (ie, from your light cube) is the best way to go.
With respect to the dark stones in the example you provided, the best way to fix the problem is to understand how light bounces around inside a diamond, and the concept of "retro-reflection". Light hitting a 90 degree corner from almost any angle will, just like corner shots in billiards, will always come back directly towards the source (eg,
http://www4.uwsp.edu/physastr/kmenning/images/retroreflector.diagram.gif ). This is what happens with the interior reflections in a diamond when viewed from directly above the "table" (ie, upper flat face) of the stone. The apex or "culet" angle is usually a bit larger than 90 degrees, so you don't get perfect retro-reflection, but the main aspects of retro reflection still apply.
If you are photographing a diamond from directly perpendicular to its "table" (ie, the situation in your example photo), and the area around the camera is dark (ie, no yellow rays in the diagram I cited above, only red and green off axis illumination), the diamond will look dark. The "fix" is to use a ring light (or even a beauty dish) to supplement the general ambient illumination plus any off-axis point sources that you may have. Balance the exposures from the three sources, and you'll have a great looking stone (or stones).
I'm not familiar with the light cube that you mentioned, but most light cubes have one missing wall so that you can photograph through that side of the cube. If your light cube is configured like this and the main light source is far (in degrees) from the optical axis of the camera, it would fully explain the problem you encountered.
HTH,
T