Caustics are those patterns that light makes as it bunches up from reflections and refractions. The most typical example is when you use a magnifying glass in sunlight to focus the light in a small hot area. Or the curved sliver of light you see reflected from things like metal rings. Light patterns seen at the bottom of a pool of water is another example.

To get them in 3D, you need a renderer that simulates those rays in the form of light photons. Poser (AFAIK) doesn't as yet have this feature so you cant really get the look of diamonds exactly. Most you can do is get a render with nice refractions and reflections and then paint the caustics in photoshop.