The scriptures leading up to the mark of Cain are important to examine in order to get a proper understanding of what God's word does and does not teach. In verse 9 of Genesis 4 God asks him where is his brother Abel. His now well-known response is 'Am I my brother's keeper?' God then confronts him with his grievous sin.
'Why have you (Cain) done this terrible thing? Your brother’s blood is crying out to me from the ground, like a voice calling for revenge. 11 You are placed under a curse . . . (Genesis 4:10 - 11)
What did God spare the life of Cain instead of taking it? One reason is that he had not yet given man a command not to murder another human. Such a command will be given, however, right after Noah leaves the ark after the floodwaters subside (Genesis 9:6).
An additional (and likely more important) reason he was allowed to live is so that other humans would be warned of God's view and judgment on sin. If he was put to death as a punishment that death would soon be forgotten (Ecclesiastes 8:10) and the lesson of what happened lost. He is cursed, instead, to wander the earth with no permanent home. His constant wandering (due to being a fugitive), coupled with a mark he will soon receive, will be a living testament to others not to repeat the same mistake.
13 And Cain said to the Lord, 'This punishment is too hard for me to bear. 14 You are driving me off the land and away from your presence. I will be a homeless wanderer on the earth, and anyone who finds me will kill me.' 15 But the Lord answered, 'No. If anyone kills you, seven lives will be taken in revenge.' So the Lord put a mark on Cain to warn anyone who met him not to kill him. (Genesis 4:13 - 15)
Why would anyone greatly fear someone taking vengeance on them ("anyone who finds me will kill me") if, as some assume, no one else but Adam and Eve where living at the time? The Bible does not state Adam's age when either of his first two sons were born. Seth was born after the death of Abel when Adam was 130 years old. The youngest age at which the first patriarchs (Seth, Enosh, Cainan, and all those to Noah) gave birth to their first son was sixty-five.
If we, conservatively, assume Adam had his first son at 65, then that would give Adam and Eve 50 years or more to procreate until Abel's murder. A footnote in the writings of the Jewish historian Flavius Josephus states a very old tradition that said Adam had a total of thirty-three sons and twenty-one daughters. The number of Adam and Eve's children, grandchildren, great grandchildren, etc. that existed when Cain received his punishment would have been enough to warrant his concern.
Why is there a warning given that if Cain is met by someone that he or she should not kill him? It is because, in part, it would defeat God's judgment and the purpose why he was allowed the world's first murderer to live. God seems to have reserved revenge for his crime to himself, and therefore warned others they should not strike back against him for his crime.
The opinions and speculations about what was the mark have varied greatly throughout the years. Different people and groups have argued that the special sign God gave was a horn (either short or long), leprosy, Abel's guard dog, a special symbol or letter, or a letter used in God's name, a tattoo, dark skin (some feel he was turned black), His own name and even the rite of circumcision. Some think the Eternal turned him into a giant!
The truth is the Bible simply does not clearly tell us what the unique mark of Cain was. Any conjecture, therefore, regarding this issue is ultimately based on opinion and not Scripture.