We only see the stone circle of Craigh na Dun a couple of times, but its presence looms over the whole novel. Though Craigh na Dun is fictitious, picture Stonehenge and you're on the right track. And just as Stonehenge has baffled and intrigued folks for ages, Craigh na Dun helps set the mysterious tone in Outlander. Claire never quite understands Craigh na Dun, and the more she thinks about it, the more questions she raises. She notes:
Whoever built the stone circles, and for whatever purpose, thought it important enough to have quarried, shaped, and transported special stone blocks for the erection of their testimonial. Shaped—how? Transported—how, and from what imaginable distance? (2.20)
Its unexplained mystical properties are what allow Claire to travel backward in time, but that's all we know for sure about it. And considering the deliberateness with which it was most certainly created, it's safe to assume there's more to this stone circle than we understand in this book. Which is fine, because symbolically, Craigh na Dun is here to make us think mystery.