What is the name of this book?

It's a fantasy book. There's magic in it, and slaves.

The main character has her eyes replaced with precious stones, which is what gives her power to do magic and stuff. She also happens to be a slave. I don't really remember anything else about what happens in the book, but those are two pretty big points.

Does anyone know what book this is?

As far as I remember, it was a pretty thick book, and I found it in the fantasy section.