PBR Marble lighting exercise - Caustics

Marbles give the opportunity to explore refractive caustics as they are made out of glass and often exhibit many colours.  Even though they are usually made out of glass, they have a kind of liquid appearance on the inside. I thought this would provide me with a good opportunity to recreate a physically accurate marble in Arnold.

Shaders, textures and geo

To create the marble, I used 2 spheres; a small inside a larger one.  The small inside sphere has a standard shader assigned to it, adjusted to represent water.  The larger, outer sphere, has glass settings applied.   I added some Helix shapes in the centre of the marble, with a standard shader applied, but with emission values and colours.  The caustics work 

I wanted to add some unique looks to the marble and make it a bit fancier so I applied a space/galaxy texture to the inner sphere, into the transmission and emission colour. I also added a black and white version into the opacity slot of the shader to allow light to come through.  After applying all these aspects, the texture could be seen on the shader surface.  

The subtle caustics can be seen on the table surface: