My biggest problem with quarry tiles isn't the toxic substances they may/may not contain, it's their potential lack of resistance to thermal shock. Commercial baking stones/firebrick (even the cheapo ones) are specifically engineered to handle the type of extreme temperature changes involved in hearth baking. Quarry tiles are not. There's a good chance the tiles won't break on you, but, at the same time, there's a small chance they will. If you're not paying attention, a shard may end up in your bread/pizza. Pit shard against teeth, shard wins. Trust me, from personal experience, I know. This scenario is probably a thousand to one, but why take the risk when firebrick is only a few dollars more?

A lot of people gush about the unglazed quarry tiles that they've been baking with for years without a problem. All power to them. Me and my chipped tooth are sticking with tools specifically engineered for the job.