Quote - 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.

The individual shape of facets don't matter. The overall shape of the surface does. So even if each facet is flat, if they join together to form a curve (like they do in most gems) then you get caustics. The most noticeable effect in gems is the 'inner glow' they show. And this is also why most simple renders it looks like its some sort of plastic, not a gem.

eg in this (real) ruby:

http://texasvc.weblogswork.com/wp-content/images/ruby.jpg

Notice how the gem actually seems to be emitting light, and not just reflecting it. Compare this to the render you posted and tell me if it is even close ?