What is the mark of Cain?
Genesis 4:8-15 Now Cain talked with Abel his brother; and it came to pass, when they were in the field, that Cain rose up against Abel his brother and killed him. 9 Then the LORD said to Cain, "Where is Abel your brother?" He said, "I do not know. Am I my brother’s keeper?" 10 And He said, "What have you done? The voice of your brother’s blood cries out to Me from the ground. 11 "So now you are cursed from the earth, which has opened its mouth to receive your brother’s blood from your hand. 12 When you till the ground, it shall no longer yield its strength to you. A fugitive and a vagabond you shall be on the earth. 13 And Cain said to the LORD, "My punishment is greater than I can bear! 14 Surely You have driven me out this day from the face of the ground; I shall be hidden from Your face; I shall be a fugitive and a vagabond on the earth, and it will happen that anyone who finds me will kill me. 15 And the LORD said to him, "Therefore, whoever kills Cain, vengeance shall be taken on him sevenfold." And the LORD set a mark on Cain, lest anyone finding him should kill him.
The mark placed upon Cain was a distinguishing mark of some kind, but we are given no other information. It was clearly visible and unmistakeable to those future children of Adam who might think of taking it upon themselves to be an ‘avenger of blood’ for the murder of Abel.
I hope you find this helpful.