I would buy a rock for terrariums without sharp edges on a pet shop, then I would rinse it with hot water and then I would bake it. I think that's the safest option If you think about something disinfected and hygienic, and that's what I did.
I bought a piece of salte, but as cypher told me on another thread, you have to be very careful because slate sometimes has sharp edges and can chip. I used a industrial file to smooth the edges and now they are flat. I think that those on sainsbury are a little bit expensive. Mine cost 2.20 euros.
I also bought this from ikea
DYFT Adorno, piedras - IKEA And I did the same process with them. Then I put them on the potty entrance.
If you want to see some photos you can check how they look here
Let's See Your Cages (May 2016)