A book (for older children or YA) about a girl who is ‘haunted’ and/or has visions.

I read this when I was a lot younger so it might be like 20 years old. This book is about two girls and one is the new girl in school and the other has to buddy with her. It turns out that there’s something weird about her, she has these horrible visions (I think). I also vaguely remember a necklace being instrumental to the plot. The cover was blue (I remember that for sure)