One major fact that isn't readily known is that clay cat litter is strip mined. Add to that, clay clumping litter does not biodegrade in landfills.
Clay litter contains silica (a known cancer causing agent when inhaled) and clumping clay litter contains sodium bentonite, which expands to 15 times its original mass and becomes completely insoluble. Think about what that does when your cat grooms itself and the sodium bentonite gets into their digestive system.
Here is the full blog post on the subject from moderncat.
More resources for further information: