Totally ignoramus: as I see all lines are straigt lines does it possible to have curves in a daimond ?

There is no mechanical reason not to have curved lines in diamonds - especially virtual ones. I guess the facet cuts have developed over time to make sure that all the light coming from above the gem is reflected back up inside the gem to get the most interesting play of light when you look at it. Curved shapes would have a different effect, and there's nothing stopping you from experimenting with that to see what happens if you have access to a realistic renderer where you can adjust refractive indices and where you get realistic caustic effects.

Metropolis Light Transport is what I used.
...and you have to know something about the IoR (Incident of Refraction & Reflection) of the gem material.
The type of spot lighting is also crucial.

FYI - shayno posted some gemstone 3DM files a while back here:

http://moi3d.com/forum/lmessages.php?webtag=MOI&msg=4423.4

Ed

A B Carter created three Rhino tutorials on how to create the gems, and explained the cutting charts.
The tutorials no longer seem to be on the Renderosity site.

- Brian

A day did pass without any MoI experiments as I also had to do some work.

Yesterday evening, I set myself a new non-gemstone challenge of making a fork, and although I overdid the curvature of it, I think it looks fine. Starting with the outline of my favourite silverware set I have at home, I did some extrusions, divisions, curve projections, as well as a sweep to make the subtle bulge on top of the handle. All good practice in using the basic toolset of MoI.

As the fork looked a bit lonely on its own, I threw in a revolved wine glass with rose-like contents I made a few days ago.