Genesis 4:12-15
12 When thou tillest the ground, it shall not henceforth yield unto thee her strength; a fugitive and a vagabond shalt thou be in the earth. 13 ¶ And Cain said unto the LORD, My punishment [is] greater than I can bear. 14 Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, [that] every one that findeth me shall slay me. 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.

Spencer W. Kimball, The Miracle of Forgiveness, p.127-128
On the sad character Cain, an interesting story comes to us from Lycurgus A. Wilson's book on the life of David W. Patten. From the book I quote an extract from a letter by Abraham O. Smoot giving his recollection of David Patten's account of meeting "a very remarkable person who had represented himself as being Cain."

As I was riding along the road on my mule I suddenly noticed a very strange personage walking beside me…. His head was about even with my shoulders as I sat in my saddle. He wore no clothing, but was covered with hair. His skin was very dark. I asked him where he dwelt and he replied that he had no home, that he was a wanderer in the earth and traveled to and fro. He said he was a very miserable creature, that he had earnestly sought death during his sojourn upon the earth, but that he could not die, and his mission was to destroy the souls of men. About the time he expressed himself thus, I rebuked him in the name of the Lord Jesus Christ and by virtue of the Holy Priesthood, and commanded him to go hence, and he immediately departed out of my sight….*

Hugh Nibley, Teachings of the Book of Mormon, Semester 3, p.78
This goes back right to the beginning with the story of the Cainites and the Sethians. We are told that Cain became a wanderer in the earth. He went to the land of Nod. Our word nod means "going back and forth." Cain was banished to the land of perpetual wandering. He became a wanderer because of the wickedness. He conspired to kill his brother and get rich. He wanted the priesthood and all that. The Lord came and talked with him and told him he was doing what was wrong. We get all this in the book of Moses in the Pearl of Great Price. He refused to listen to the Lord anymore. He stomped on his heel and walked out on him, because Cain, his wife, and [most of] the people loved Satan more than God in the time of Adam and Eve. Adam and Eve could do nothing but mourn before the Lord. They followed Satan, took instruction from him, and got the degree of Master Mahan. The secret of this is how to get rich by taking life, how to convert life into property. When he got it he said, "Truly I am Mahan, the master of this great secret, that I may murder and get gain [Moses 5:31]." So by a conspiracy we are told in the [Pearl of Great Price] he murdered his brother Abel. He planned it all out, because Satan told him how to do it. Having done that, he said now I am free. I got the money I want. Wealth makes you free, as Mr. Forbes tells us.

Cain said, "I am free; surely the flocks of my brother falleth into my hands." So he had them. For that he became cursed. He could no longer cultivate [p.79] the earth. The earth refused her strength to him, and he became a wanderer on the face of the earth. He became a nomad and a raider.

Milton R. Hunter, Pearl of Great Price Commentary, p.170

In Moses 5:34–41 (quoted above), the story is told of the Lord's placing the following curses on Cain; first, the earth would not yield abundantly for him; second, he would be a fugitive and vagabond in the earth; third, a mark was placed on him; and fourth, he was shut out from the presence of the Lord.

A connection between Blacksmiths, Gypsy's, & Cain and his curse:
Hugh Nibley, Ancient Documents and the Pearl of Great Price, p.3
There is a great tradition about the mark on Cain. In Peter Matthiessen's book on the Himalayas, he writes an interesting thing. There's also a massive work on this by Robert Eisler. Unfortunately, I've loaned it to somebody here on the campus and wasn't able to bring it with me today. It's just as well because it would use up too much time. But it's these Cainites that wander. Cain, Qayin, is the wandering smith in Arabic, Hebrew, and Aramaic. A qayin is a blacksmith. He blackens his face professionally because he works at the forge. This is a mark of his profession, the blackened face. It advertises his profession, and he wanders. You find these, and they are great metal workers, as we will see Cain's descendants are. Their rites are secret, and they intermarry. You think of the Gypsies, of course. The Gypsies belong to that particular class of people. They are always wandering. We don't see Gypsies as much as we used to. There used to be a lot of them. When I grew up in the Northwest, we had Gypsies with us all the time. They would park in the back lot, and they were remarkable people. They were the Romany. Gypsy is related to Egyptian, but they spoke a language very close to Sanskrit. Maybe you have read George H. Borrow's Lavengro. That used to be required reading from everybody. It is about the boy who grew up among the Gypsies and learned the Gypsy language and secrets. They have Gypsy princes. For us they are rather a sinister people. They live to themselves, and they don't hesitate to steal or anything like that because that's legitimate. They can't make a living. The earth will not yield. They cannot plant because they say it won't grow if they do. They tell fortunes and have all sorts of insights. They can really tell them too. I've had some beautiful fortunes told, and they hit it "right on the button."