Symbolism, Imagery, Allegory
In The Fallen Star, a lot of the characters have marks on their skin. At first Gemma thinks that they all have tattoos, but Laylen soon explains to her that these are marks that appear all on their own—no visit to the tattoo parlor needed. When Aislin shows off her marks, she explains:
"What is that?" I asked, pointing at her shoulder.
She traced the lines of the tattoo with her finger. "It's the witches' mark. After I became a witch, it appeared on my skin." (20.31-32)
The "tattoos" signify characters' memberships in certain supernatural groups. As a vampire, Laylen has the mark of immortality, whereas Aislin has marks that show that she's a witch and a Keeper. And when Gemma sees a vision, the mark of a Foreseer starts to show up on her skin:
"Well, it looks like you got your mark."
"A Foreseer's mark?"
He nodded, tracing his fingers gently along the back of my neck. "I can see the outline of it forming right here." (31.73-75)
Beyond supernatural membership, the mark symbolizes to Gemma that this is all real and that it is truly part of her fate to be magical and save the world. She just has to accept it. It's literally writing itself on her body, after all. And in this way, the marks also indicate that as much as they announce people's various powers, people are also powerless against their powers: They have no say in whether they have them or not.