Joined and submitted this 3D crystal / diamond / quartz render for the Blender3D Facebook group challenge. I have written a little bit of info on how is this made, including the render settings and shader node for the crystal. Not really a full fledged tutorial, but it is more than enough to use this as the foundation for your own project.
ArtworkBelow show the difference between Cycles and Blender Internal Render.
You can also enable "Extra Objects" plugin in Blender3D to add Gems/Diamond mesh easily.
ReferenceDeviantArt Blender3D Crystal Shader Tutorial
DeviantArt Blender3D Internal Render Crystal
DeviantArt Crystal Collection
DeviantArt Blender3D Cycles Render Crystal
DeviantArt Blender3D Hidden Gems and Diamonds