Can you sell a crystal at a jewelry store?

Unless you're an experienced gemologist and know what you're looking for then no, there's no way of telling whether the diamond you're seeing in a jewelry store is real or if it's crystal. The only way of ensuring you do not get ripped off at a jewelry store is researching the store you're thinking of purchasing your diamond jewelry from beforehand. Choose a reputable jewelry store and you won't go wrong.

And don't forget to ask for the diamond certificate which should come attached with every piece of diamond set jewelry. If the piece does not have an attached GIA or AGS approved certificate of authentication then I would avoid buying it altogether.