On what were written the verses of the Quran when they were revealed to prophet Mohammed?

They weren't written down until later, when Mohammed recited them to some designated scribes. Mohammed couldn't read or write, so he would not have been able to review them for accuracy. The original Quran is not extant, so no one knows for certain what it says and there is no way to prove it. It was probably written down on papyrus or vellum.