Many of these ideas are interesting, but we'll have to continue our search to find out what was the real mark of Cain.
Temple symbols in Genesis
It is important to understand a key aspect about the narrative in this early section of Genesis. It is that the writer of Genesis was totally temple-oriented. All his terminology and geography have temple symbolism associated with them.
For example, note that "in the Garden our first parents were able to talk face to face with God."
But also note that they had conversations with Him only at certain times of the day. They did not see Him on all occasions. It was "in the cool of the day" that they came into "the presence of the Lord" (Genesis 3:8).
The expressions "cool of the day" and "the presence of the Lord" were a part of temple language.
The cool of the day was the period when the sun was lower in the sky and the cool sea breezes normally swept over Israel.
This was the time of the evening sacrifice (1 Kings 18:36; Daniel 9:21), about 3 in the afternoon.
At these times the people were reckoned as being "in the presence of God" (2 Chronicles 20:19).
The phrase "in the presence of God" is just one example of the many that could be given to show clear temple symbolism in the early chapters of Genesis.
We must understand that the writer, Moses, is orienting readers of his days to what was taking place in the ancient period to reinforce the system he was introducing at that time. Recognition of this fact can help to provide the keys to identify what the "mark" was.
When we orient ourselves to the fact that this narrative is illustrative of temple symbolism and even geography, we can start to piece together what the mark was.
First it is important to understand that Cain was banished to the land of Nod. He was removed out of "the presence of God."
The land of Nod lay in a particular place relative to Eden, on its eastern quadrant.
Rabbi Solomon ben Isaac noted that Cain's removal was to the land of Nod and that a city was built there that was typological of the future, when Moses would establish cities of refuge for murderers east of the tabernacle.
In addition, we find the early Jewish legends (Hebrew midrashim) talking about Cain, saying:
"Rab said: He [YHWH] made him [Cain] an example to murderers. Rabbi Hanin said: He made him an example to penitents."
Could it be that Cain was given this mark as a form of penance so people seeing him would recognize him as one in a continual state of repentance for his deeds?
There is an interesting comment in the Word Biblical Commentary without explanation that could (coupled with the previous statement from this Jewish midrash) provide us with the answer.
It says: "The mark of Cain must be something about him that shows he has divine protection . . ."
The point then made briefly by Gordon Wenham is that some have suggested that Cain may have had a "special hairstyle."
Could it be that, by looking at the special hairstyles worn by penitents as outlined in the Old Testament, we may have found our answer to what the mark of Cain was?