Don't use the crystals! We (euh, my cats did) try that for a while. The smell in the neighbourhood of the box is terrible: like amonia! The "used" crystals turn into bright yellow crystals, yuck... Not very appetising to see those!!
We use clumping litter from "Vitocraft". It is expensive, but the best! To get rid of it, we can't flush it down the toilet , it goes with the trashbags!