It is not know what the mark was on Cain. The Hebrew word means 'sign', and this could have been either a physical sign or a miraculous display of some kind. In either case, it served to protect him from being murdered.
Jewish AnswerThe mark on his forehead was the letter 'vav' which is HaShem's mark. Depending on what is used to make the mark, it's either HaShem's protection or condemnation.
Side fact:
The Christian "mark of the beast" comes from a misunderstanding of the discussion of HaShem's mark in the Tanach (Jewish Bible). The letter 'vav' has the numerical value of 6 and that's where the Christians created the concept of '666'.
God set a mark on Cain because God had mercy on Cain. After pronouncing Cain's punishment, and hearing Cain's reply - "My punishment is more than I can bear.", God put a mark on Cain - Genesis 4.15. And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.
The mark of Cain is mentioned in Genesis 4:15 in the Bible. This passage describes how God placed a mark on Cain to protect him from being killed after he murdered his brother Abel. There is no specific chapter in the Bible referred to as the "Book of the Damned" relating to the mark of Cain.
The mark of Cain was first introduced in the book of Genesis in the bible. God cursed Cain and put a mark on him after he murdered his brother Abel.
Ill at Ease - The Mark of Cain album - was created in 1995-07.
God put a mark on Cain but the Bible does not say where or what type of mark God put on him.
Gunsmoke - 1955 The Mark of Cain 14-19 was released on: USA: 3 February 1969
Our Family Honor - 1985 Mark of Cain - 1.8 was released on: USA: 29 November 1985
Lawman - 1958 Mark of Cain 3-28 was released on: USA: 26 March 1961
Petrocelli - 1974 Mark of Cain 2-2 was released on: USA: 17 September 1975
The Untouchables - 1959 The Mark of Cain 2-5 was released on: USA: 17 November 1960
The Mark of Cain - 1947 is rated/received certificates of: Finland:K-16 UK:A (original rating) UK:PG (video rating) (1997)
Have Gun - Will Travel - 1957 The Mark of Cain 5-18 was released on: USA: 20 January 1962