I was just trying some weird stuff and thought I would share. I thought it was pretty cool. I’m sure someone else has already done this but some people may not be aware. It may be a little difficult to see. Taking a picture of a hidden message under UV light is trickier than I thought.
Very interesting - are you just engraving the tile or are you applying a coating and ablating the coating with the laser?
Since you’re playing with UV I’m going to guess Zinc Oxide. 
I apologize for the lack of specifics in the original post. Its actually just a piece of manilla folder. I was trying different materials for fine focusing and a voice inside my head (I hope I’m not the only one who has one (or more)) said, "what if you shine a UV light on it?? Hope that answers any questions you may have. I had also considered attempting to make a Marauders Map from Harry Potter. My wife is an art teacher and a huge fan and I thought her students would love it. Unfortunately it never came to fruition. I was aging the paper because it looked a little ridiculous as a plain manilla folder. However, aging the paper removed its UV “ability.”