Christian tattoos.  Here is a profile of a Coptic Christian family in Jerusalem that has been tatooing crosses on the wrists of Christian pilgrims to the holy land for several generations. I don't have any tattoos (so far); for years I've said that the only way I would get one would be if I visited the holy land, in which case I would be sure to visit these people.

Egyptian Copts usually wear a small tattooed Cross on the inside of their right wrists; these tattoos offer protection, a statement of commitment, and a means of identifying one another in a context of severe discrimination. 

In effect, the small black cross tattoo that virtually every Copt wears is a visible reminder that in an overwhelmingly Muslim society, they represent the “other.”
They’re not just a symbol, but also a form of ID. For safety reasons, many churches station security personnel at their doors to check that those entering have the tattoo as a guarantee that they are in fact, Christians.

Conquerors of Christian lands hated images of the cross, of Christ and his saints; sometimes they scratched out the eyes of icons, in a futile attempt to eliminate their power. So I want my faith to be visible to the world. For some that might mean wearing a prayer bracelet, wearing a cross visibly, a bracelet of small icons — there are many ways to silently bear witness to your faith. Whether times are going to get harder or insidiously softer, bearing visible signs of our faith will keep us aware that our first allegiance is to Christ. They will remind us and our fellow Christians, and bear witness to powerful despisers, of where our first loyalties lie. They will remind us of the decisions and commitments we have already made, in the sacraments we’ve received and the worship we offer, and strengthen our conviction that nothing will turn us back.