Is the mark of Cain a physical mark or is it a "pointing out" of a spiritual principle?

What do we know about Cain? Cain was making an offering to God much like his brother Abel. Abel's offering was meat and Cain's offering was plants of the field.

Cain became jealous when God did not accept his offering, and slew Abel, his brother.

Cain was then "marked" by God, and nobody was supposed to hurt Cain.

Here, in the lesson of Cain and Abel, we have many things about having faith without having a good heart.

Cain's offering of plants, and Cain's murderous jealousy both point to faith without charity.

Yet, Cain is not to be hurt. The Lord has "marked out" faith or pointed out that we should not hurt someone's faith, even if it is yet apart from charity or goodness of heart.