Do you recognize this sci-fi young adult book?

This is a book I remember reading probably 10-15 years ago. But I don't remember what it is called. Some help?

It is set in a time when the world has gotten very cold. There are 2 social groups: the elites and the slaves. The slaves work in the mines to get enough of the 'firestones' (I'm not sure if that is the correct name) in order that everyone can survive.

The main character is a young slave girl...maybe 16? Like all the other slaves, she was branded with a flame in a circle on her forehead when she was a child. She gets chosen to be the handmaid/assistant of the man who has the power of being able to find new mines. He is taking a social risk by choosing her, and is trying to improve the lot of the other slaves.

The story follows her travels with him, and the abilities she develops along the way.

Ring any bells?