How high is the "cliff" ? In general, stone has great compressive strength but is unreliable in tension. An unsupported span puts the stone in tension when it is loaded. A 4" thick piece of granite might support a ton with a span close to 3', but it might crack in half the first time somebody jumps on it. I don't know what type of stone your mason would use, but I would suggest either supporting the stone underneath or using a different material. Unless you know a lot about the stone and test each one before hand, you would have some risk. Recently, I had a beautiful 5' long piece of 3" thick granite ruined on a job site when a careless worker dropped a 10 lb rock on it. It cracked in half.