Silicon Dioxide (silica) is a constituent part of the chemical composition of the clay (Al
2O
3 2SiO
2 2H
2O) from which the quarry tile is formed. (Incidentally, roughly 60% of the earth's crust is silica, aka, "sand.")
I hesitate to point this out, but if the possible toxicity of silica is a concern, exposure from baking with quarry tiles should be the least of your worries.
Silicon Dioxide (SiO2) is a common
anti-caking/anti-clumping agent, so if you're using just about ANY powdered ingredients, dried spices, dry mixes, grated cheese,
etc., you're already ingesting non-trivial amounts of silica. Not only that, but the dessicating packets commonly found in,
e.g., pepperoni pillow packs, vitamin and prescrption drug bottles,
etc—the ones that say "Do Not Ingest"—are packed with silica gel.