Scarstone

Scarstone is fantasy spun from the real world of the Iron Age Celts, the quest of an outcast girl to master her magic and heal her broken family. Magic is mistrusted, and Wynn’s, even more so, given the suspicions cast on her absent father. Though untutored in the proper purpose and use of her magic, Wynn tries to forge an object of power that will set her above them all, but instead, she burns down the smithy.
The punishment for causing such a devastating loss is death, so she runs away.

But that disruptive use of magic reveals her presence to Nathir, Archdruid of the Order of Naddreds. Locked in Scarstone by her grandmother, the last Magewoman of Awe, he needs one of her blood to release him. He has long lusted for the power of the crystal that only she can wield. When she flees, Wynn encounters an elf that appears to be her age, but is old enough to have fallen in love with her fabled grandmother half a century ago. He knows the way to a capricious water deva with knowledge of the crystal Nathir covets. Theft and death force a disheartened boy to become an ally, thought he shuns magic and cares only for his horses.

Once they find the deva, her riddles lead them to the crystal, the secret of Scarstone, and the knowledge that their pasts and destinies are entwined as a Celtic knot. Poised to destroy Scarstone and her enemy with it, the girl finds that her mentors have shaded the truth, that she must relinquish one good to hold another, and that her flaws are the only amulets left for her to wield.