They seem too dark to me. And the dispersion seems exaggerated.
Mine always come out bright and the dispersion is hardly noticeable. But, I stick to 'real' settings.
Another question, is your model accurate? The 'brilliant cut' shape is very precise and is designed to make a bright diamond. Before the advent of proper cuts, diamonds were dark, sometimes almost black. Don't know how much you know, but I've spent many an hour studying pictures of real diamonds and reading about the history of various cuts in search of making the most realistic diamonds I can. The articles on Wikipedia are a pretty good start.http://en.wikipedia.org/wiki/Brilliant_cuthttp://en.wikipedia.org/wiki/Diamond_cut
For pictures, just use Google image search for "diamond".
My attached image was inspired by yours. It was rendered to 5kS/p using Lux v0.8 and Blender 2.6 and took about 20-1/2 hours on my Core2 Quad 3.0Ghz (Q9650).
model: 3DLapidary - http://www.3dlapidary.com/
- standard round brilliant
hdri: HDRLabs - http://www.hdrlabs.com/sibl/archive.html
- "Charles River Esplanade"
Hope my input helps.