You could use any natural tile or granite or marble, ceramic and/or porcelain tiles would be good and you wouldn't have to worry about anything leeching into the water.I don't think any chemicals are used to make them. You may need something to secure them to the back wall so they don't fall over,other than silicone,I am not sure what you could use.
I have often thought the small natural stone tiles would look good in an aquarium as a background or partial background.The same type they use on fireplaces.
The only ones to worry about would be limestone tiling,and any with
calcium,sandstone,or such in them.They would alter the
pH over time,but in some aquariums may be beneficial,by adding buffers as they leech out into the water.Not good for everyone,but for some it wouldn't hurt.