The basic idea, using a glass shader for outter shell and an inner sphere with textured volume to fake depth. this way, you will save a lots of work with actually modeling the inner part of a marble. Spectral renderer don’t use this kind of trickery they can actully the inner part as real glass medium (using textures).
I’ve use Cycles (Blender), Renderman, Arnold, Octane and Indigo Renderer. I’ve tried to create a Marvel in Redshift, but i could make it work with single texture and 2 spheres. For Redshift you need actually model a marvel to get realistic rendering.
the spectral rendere engines was the fastest by far. that’s because with spectral render I used a medium instead of volume for interia, that’s saves a lot of render time.
here is quick test with single glass object with Indigo and Cycles :
this here are glass spheres with regular solid texture spheres inside: