Diamond or Glass

I used to think a diamond in the rough was a diamond chip that was lost in the grass - maybe I equated it with the image of "a catcher in the rye." Then I went to a jeweler's with a friend who said, "show her the rough diamond!" The Jeweler showed me a diamond in the rough. Looked like a piece of broken glass, slightly worn down. How could she tell it was a diamond? She knew how to look at it.