(At this time I am actually up in the air on my way back to Vancouver but I have been saving this post for just such an occasion.)
This is a marvel effect that I achieved by accident while I was working on another project. I was cutting through dark coloured sheets of raw clay veneers and decided to us a layer of translucent and white scrape clay underneath to make clean up easier or some reason that I can't recall at the moment.
As a result I embedded thin lines of colour in the translucent/white clay underneath. When I ran the this clay through the past machine a couple of times it really looked like marble. I cooked up a sample and the result still had a real marvel effect.