And I have deduced that the author's last name comes after Warner. It's about a slave girl who has a brand on her forehead who is working in a mine. There are a few people who can sense the material they are mining for and a man comes, recognizes her as one of these rare people, and takes her to train her. But before he can deliver her to the leaders he dies in a snowstorm. She travels onward and when she reaches her destination they treat her with great suspicion. They test her by taking her to a barren field where the material has been strip mined from. She is followed by a large group of judges. Not finding the material she gives up and passes through the crowd. Then discovers the one man holding a piece of the material. All the other judges are holding fakes. And that way she proves she's the real deal.
Do you remember how long ago you read it, and whether it was a young adult, adult, or children's novel? Some of the plot elements sound like Winter of Fire, by Sherryl Jordan, although the author's name does not come after Warner: http://www.goodreads.com/book/show/118755.Winter_of_Fire Omaha Public Library no longer owns a copy of this book, so I can't check to see if it includes the scene you describe, but if you think it might be the right one, you could request it via interlibrary loan.
Let me know if you can think of any more details, and I'll be happy to look further!