There is a gemstone material that works great. You can adjust the Abbe number to get the wanted refraction effects.
Also, remember to turn on caustics in your environment for extra effect.
Not achieveing desired result. Diamond looks more like a glass crystal no refraction @ abbe 98
Besides adjusting the Abbe number, try also adjusting the IOR. The higher the number, the more refractions inside the diamond. I think the physically correct IOR for a diamond is 2.6. You can even google search these for all materials in the physical world (ie: water is 1.5 IOR).
In my experience, the lower the Abbe number, the more Chromatic Aberrations (color bounce) you will see inside the diamond.
Also, for jewelry, one of the main things that make/break your render is the HDR used. Jewelry is mostly very reflective metal and extremely reflective gemstones. Therefore, the HDR drives much more than the lighting when rendering jewelry, it's also drives the reflections. So try to chose an HDR that has some good contrats (black black and white whites) like the Blurred Photo Studio. I think the emerald ring example movie uses this HDR.
One final note, since HDRs are so key in jewelry renders, you might want to consider buying a separate product, which Visualize supports a great plug-in. This is HDR Light Studio. It's not too expensive, but opens up a whole new world for lighting all your Visualize product. I love it myself. Check it out here. And see an old video produced in Bunkspeed with this workflow.