Hi. Regular slate is fine (not sure what would be considered "artificial slate"). Just clean it as you would anything else before putting it in the aquarium. I usually soak rocks in a mild bleach and water solution followed by a good scrub and rinse.
How thick is the slate? If it's like slate tile, you can use a wet saw. But you would then have rough edges and need a dremel (or something) to round it off.
I had large lumps of plum slate I was trying to destroy recently to add to my tank. A rubber mallet and chisel weren't doing the job, so I used a small sledge style hammer on its own, with a lot of large swings. It crumbled into lovely pieces.