Quarries, of course, but forgetting that, as another poster
pointed out, Special Kitty cat litter, 100% pure bentonite clay.

Not all bentonite is the same. There is a sodium and a calicum type. It is
the latter one wants to use, most kitty litter is the former. I think there
is an article in the latest Water Gardening on this. ~ jan

The main difference between the two is one clumps, the other doesn't.
According to what I've read.

C//