I believe that caustics aren't the main issue - a flat facet will not form them - caustics are created when a curved surface focuses otherwise parallel beams of light onto  a point.

What is really important is the total internal reflection (TIR) effect. This is when light that is inside the diamond (which has a very high index of refraction) cannot pass through a facet when hitting it below a certain angle. As a result, instead of leaving the stone, it reflects back into it and usually comes out the front, or at least it does when the gem is cut at the right angles.

If you follow the link that kalon gave you to my thread on the fresnel effect, you'll get close with just using the reflect refract tricks I show there. Those are the kinds of shaders I have on these gems in this render. But the TIR needs some help as Poser won't do it. What I did here was after importing these gems from the lapidary site (listed above) I set up two material zones. The top zone is using my colored glass shader. The bottom zone (for the facets that face downward) is just 100% reflection. If you look at the back of these gems, they look like perfect mirrors which is not accurate. But if you're rendering the front of them, then what you see inside the gem is total reflection of the scene visible through the top of the gem.

Diamonds also show "fire", which is caused by the prism like splitting of light into a rainbow. You can get some of that effect as I did here by setting up a Color Ramp node with some pastel green and red in it and drive that with an Edge_Blend node. I'm showing that in the screen cap above. Thus you see some purple and green in the diamond reflections.

Because 90% of the object is basically reflections, you must set up things in the scene to reflect. Here I'm using a blue sky dome surrounding the entire setup. That's why some of the facets, especially the top of the ruby, look blue. You're seeing the direct reflection of the blue sky.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)