http://vector.tutsplus.com/tutorials/tools-tips/how-to-create-3d-gemstones-using-adobe-illustrator-and-google-sketchup/

Most people think of Google’s Sketchup (Free and Pro) product to be primarily a 3D tool, but it is also a powerful means for creating complex shapes to be used in 2D vector packages. In particular, it is a fabulous tool for creating primitives that require precise geometry in order to “look right.” We’ll be creating a gemstone in this tutorial. I use Illustrator and Sketchup Pro, but similar effects can be achieved with Corel, Inkscape, Xara or other tools and the freeware version of Sketchup.