Baking Steel sell their own bricks. A food-safe pumice grill brick is the same idea: knock carbon and rust off carbon steel, then wash, dry, oil.
Use it on a cool steel. Do not gouge a cordierite stone with it — that is how you buy a new stone. After rust, re-season: thin oil, 190–200°C (375–400°F) for an hour.
Buying guide: How to clean a pizza stone, steel, and pizza oven