What was the mark of Cain, and what purpose did it serve?

In the Hebrew it is just 'ot,' a sign or mark, so it is not definitive, and it has been a guessing game as to what it was. There have been all kinds of guesses, some bad, like the idea that it was dark skin that started the negroid race, which is a stupid answer. My own opinion, and I have never seen this before, is that it was the Hebrew letter for the number seven, zayin, written on his back where he could not see it but knew it was there. The letter zayin looks very close to our number seven, except it has a little tail on the end. Why seven? Because God said whoever kills Cain will suffer vengeance sevenfold. So if someone came to kill Cain, he could turn around and show them the seven on his back and say, I could not have drawn that there, I cannot reach my back, so God had to put it there, and He did it because whoever kills me will be punished sevenfold. It may also serve a double meaning, because seven is the number of the Sabbath, of rest, of forgiveness and salvation all over Scripture, so the mark could remind Cain that God is still the forgiver, the giver of rest, the one you go to renounce your old life and start over. That is my opinion, and take it for what it is worth.