"What was Cain's mark mentioned in the book of Genesis?"
This enigmatic question has puzzled many people for hundreds of years.
The word "mark" in Genesis 4:15 in the King James Version of the Bible is a translation of the Hebrew word "owth". According to Strong's Exhaustive Concordance of the Bible, the Hebrew word owth may be rendered "mark", or "signal", "flag," "monument", "sign," or "beacon." Young's Analytical Concordance renders this word "sign." The Critical and Experimental Commentary by Jamieson, Fausset and Brown shows that this word can be translated "sign," "token" or "pledge."
The Hebrew word "owth" could indicate a boundary marker that God set up to separate or segregate Cain and his descendants from the rest of mankind. Genesis 4:12, 16 shows that God banished Cain to the land of Nod — meaning land of "wandering" —east of Eden or the Holy Land. The owth — mark or sign — that God made for Cain could also have been something to carry on his person , perhaps around his neck — from which the ancient and modern custom of wearing a charm or talisman for protection may be derived.
The Jewish Publication Society translation renders Genesis 4:15, "And the Lord said unto him [Cain]: 'Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold.' And the Lord set a sign for Cain, lest any finding him should smite him." The original Hebrew conveys both meanings — a mark on the person of Cain, or a sign set up to segregate him from others.
Some have thought that Cain's mark involved skin color. Cain's mark was not the changing of his skin color. Cain, in accordance with certain West African traditions, may well have been dark, but his sin was a matter of character. God created ALL races — white, yellow and black, to serve and glorify Him with their own particular abilities. God intended races from the very beginning by putting within Adam and Eve the genetic material to begin various races. Racial differences are not due to the "mark of Cain."
Remember God "hath made of one blood all nations of men for to dwell on all the face of the earth ..." (Acts 17:26). And He is no respecter of persons (Rom. 2:11).