Eirwen by Jesikah Sundin

All hearts are capable of breaking.
Especially a heart of stone.
Eirwen Blackvein grew up in a dwarven mining camp at the edge of the map. A perfect place to hide an elven princess destined to destroy a queen and save a kingdom at war, as foretold in a seer’s magic mirror. Except, Eirwen doesn’t know she is the lost princess. Only that she was found as a newborn with her heart carved out. And why she now possesses a heart of stone in her chest, one forged by the dwarves.
A heart made from the black stone she mines to power war machines for her realm’s king.
The same enchanted stone coveted by a neighboring queen to destroy an entire kingdom.
Eirwen’s only desire is to protect her workers from the soldiers who are seizing mines for the crown. Even if it means kidnapping Lieutenant Prince Florian Halivaard, the youngest son of a warmongering king she discovers half-dead outside of the Dark Forest.
With monarchs from different kingdoms willing to kill for her heart’s magic, Eirwen finds herself at the center of a losing battle. Her only chance to protect those she loves and defend her kingdom is to trust her enemy—a prince with haunting silver eyes fated by the faeries to one day meet a heart of stone.
A heart he will break.