Diamonds distort the light that passes through them, and this is why if you try to read printed text through a real diamond, the letters will look blurry. If you try to do the same with a rhinestone, the text will be more easily readable (close to how you would see it through glass). Keep in mind, however, that diamonds that are cut shallow and have parallel walls (e.g., emerald-cut or baguette-cut stones) may not distort text as visibly, making this test inconclusive.
http://www.cyrhinestones.com/shaped-rhinestones/