Not caustics, and nothing faked. It was is a straight render, then in Photoshop;

-Used Bryce distance render to put in a dof (gaussian) blur of 4 pixels in the background, 25% sharpen in the foreground.
-Adjusted the images levels; increased the white & black levels.
-A little grain.

Sometimes...you can get a very slight caustic looking effect, but it is only the light/shadow that passes through the sphere, and since the shadows are soft, it kinda/sorta/sometimes looks something like a caustic.

Sorta like this B5.5 render; Bryce Blues

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