The more decent shot with slightly misangles  light on the central diamond but focus a bit off is the problem here.  I will post one with a "dead stone" but that will be later when I locate the pic..likely from work. Then you will see just what the problem really is.

Light tents, etc. do work fine especially with paper things moved around to control reflections..on metal, that is.  Still, the stones will come out dead and not sparkle.   The led and fiber seems a good bet.

This shot was taken with a flourescent lamp with tissue as diffuser(down and dirty work!).  Ambient room light lit the stones and fortunately did not reflect too much on the metal.