Warmth question

I don't have a book to discuss, the book topic above is just because i couldn't post this without a book title. I am looking for a book i read a long time ago. I can't remember the title or the author. I do remember the story. It's a Fantasy book about a girl her people are enslaved and forced to mine coal for some reason the land is cold and the only way to heat it is with coal. The girl's people are branded on the forehead so they are easy to identify. When the girl is branded she fights them and the brand slips so her brand is different. what is the name of the book? something to do with light or warmth
Please help