Witchmark

A Witchmark was believed to be an item that could only be manifested by Witches. What, exactly, the item was varies depending on who is speaking of it. The most common belief is that it is some form of smooth, shiny, black stone. It will appear to a Witch to signal that things are about to take a turn for the worst. Some believed that you would be able to look into a Witchmark and be able to see your own future. Of course, if this were true, you would be equally likely to see your past, due to the nature of Witches and their relationship with time. Once Witches became more common in the 3800s, emerging into public view as something that truly existed, a black, shiny stone on a bracelet or necklace became a symbol that you either were a Witch or that you were a Follower of the Void.
Fake Witchmarks have been sold, of course. It isn't that hard to find a black stone and polish it to the point that it would trick any non-Witches. Real Witchmarks, on the other hand, have never been documented to exist. Some Witches mention having had one, but these stories have never been confirmed officially. That said, Witches are not often cooperative with attempts to study them through a scientific lens, which could be why these things are so hard to confirm.
What Witchmarks are actually for is unknown, assuming they exist. It seems odd that they would appear as an omen. Given everything that we do actually know about Witchcraft and the Void, it seems more likely that there is some actual use for these stones. Unfortunately, Witches remain some of the most stubborn and cryptic beings to exist.
I was once able to convince a Witch to speak to be about his Craft and his views on the world. When I asked him about Witchmarks, he only laughed at me and did not actually answer my questions. This appears to be a continuing trend. It is possible that Witchmarks are some form of inside joke that the rest of us simply aren't in on.