Most conventional cat litter is made from natural clay, or sodium bentonite, which is formed into pellets and dried. Bentonite is strip mined from the earth in an extremely destructive process, where layers of Earth are removed and plumbed for useful materials. While the landscape is supposed to be restored after mining, the minerals and metals have already made their way into ground water sources, rivers and streams. The land will take years to recover.
When sodium bentonite is disturbed while pouring into cat box, cleaning catbox or disposing of waste, silicon particles that are a known carcinogen are released. In addition, clay is not biodegradable.
There are many alternatives. The clay-alternatives fall into several categories: wood-based (pine, cedar), plant-based (wheat, corn, grass fibers), and paper-based (newsprint, recycled newspaper). Unlike clay, most of these litters are biodegradable.
I have tried most of the the alternatives. I began the transition from clay to pine based litter about six months ago. My cat does not use the catbox as much. I do not know if it is the feel of the wood, the smell of pine, or just change. Its hard to determine because, at about the same time, I put a special cat door in the window. I think my cat may prefer to do her business au naturel.