BurrMan, until I do make a tutorial, I did find someone else online who use a method very similar to the one I devised. His method is described here: http://www.3dlapidary.com/HTML/Facets.htm and it should work well in any 3D program.

The website also has a lot of finished gem files, though some conversion might be necessary. There are Rhino files there, which supposedly should work in MoI, but when I tried opening them, nothing showed up on my screen. Not sure if there's a version mismatch as some of these seem rather old. I have started converting some of the .obj and .dxf files, though. As much fun as it is to make these oneself, it is time consuming (though, that is perhaps part of the fun too as long as you consider it a hobby), and it's nice to have a collection of finished gemstones on hand, too, for other jewellery work.