What's going on inside the diamonds fits the definition of caustic. Currently and for the past 5 years or so most CG people only use the term to refer to the light that passes all the way through the object and is projected, reflected or in other ways perturbed onto another object. A great example on American Televison right now is the Visine "water drop" commercial. The light that passes through the water drop becomes this magnifying-glass-burn of light that dances around as the droplet changes shape. Simple, silly, but really beautiful. AND it gets the red out!