Normal porcelain and ceramic tiles are 10mm thick and are baked to harden them in the manufacturing process.
Granite, or any natural stone tile is normally 20mm thick. It is hewn from a quarry, and any thinner would be prone to break along the seams of the face of the tile.
However, you can now get tiles that have a 5mm face of granite, on a 5mm porcelain back, which are fine for floors, and have all the strength of porcelain, but are the same as granite.
If the tiles you have are only granite, with no backing, then I would question thier suitablility as floor tiles. They would be fine for walls (in fact 20mm granite tiles would probably be to heavy for walls).
(But I'm no expert also, this is just personal experience).