I've been experimenting with marbling fondant because why not, but I tend to make it too dark and mix it too much. I came across this method used with polymer clay to mimic abalone and thought that it could work with fondant if it was adapted a little. Here's the video, and if you just want to look at the photos those are below the video so you can do a quick visual scan of how I went about it.
No time to watch the movie? Here's the quick rundown. The important things are to use contrasting colors and to freeze the block before cutting it so that it doesn't get distorted.
Slice pieces off and this is what you get:
Kara Buntin owns A Cake To Remember LLC, online cake supplies at www.acaketoremember.com and www.acaketoremember.etsy.com