It's been about ten years since I read this story. It was a fantasy story in a sci-fi and fantasy collection of top stories. The story followed a boy who met a girl, and I don't remember much else except that it turned out the girl had a jar full of... finger bones? baby teeth? Some other substance? that couldn't be replaced and that gave her magic and/or protection of some kind.
For some reason, the boy decided to smash the jar, and the girl either died or lost her powers in a big, emotional moment.
I'm pretty sure it was in a short story collection alongside the story "Learning to Be Me" (Not in the book Axiomatic, however), but I could be very wrong. I know I read both of those stories very close together, and I think I only had the single collection of sci-fi and fantasy stories.