Not sure when the book came out.
It's about a girl with an acorn in her eye, but she doesn't know that until later in the book when it comes out and with that eye she can see magic in the earth. The girl, I think, was a teen and she was on a quest to go somewhere and people were trying to attack her.
I don't remember much else about it, including what the cover looks like.
Witchy Eye by D. J. Butler.
Sarah Calhoun is the fifteen-year-old daughter of the Elector Andrew Calhoun, one of Appalachee’s military heroes and one of the electors who gets to decide who will next ascend as the Emperor of the New World. None of that matters to Sarah. She has a natural talent for hexing and one bad eye, and all she wants is to be left alone—especially by outsiders.
The summary doesn't mention it, but Sarah does have a magical acorn in here eye. She mentions it in this passage:
What did her own aura look like? As discreetly as she could, she gazed down through her witchy eye at her bare hands. She shouldn’t have been, but in the moment, she was shocked to see that her own aura was blue-tinged, more like Thalanes’s than Calvin’s.
Like the acorn that had fallen from her eye.
She pulled the acorn from its hiding place and held it out to Thalanes with a trembling hand.
He looked puzzled. “What’s that?”
“This is what was troubling my eye,” she said quietly. “This was inside my eye, under my eyelid.”
“I’m not sure I understand,” he said.
She tried again. “It’s always been inside my eye, without my knowing it, and today it popped out.”
There was no special trick to find this, just brute force Googling. I eventually spotted it in the result from the search "acorn in her eye" magic earth fantasy novel.
