Did you know that Chiarman Mao's coffin was built with Hainan crystal? So it's no surprise that Sanya is famous for its crystals. Everyone here also says that crystals have health benefits, if you believe in that sort of thing.
Since convincing crystals can be made from glass or plastic, there are a few local tricks to see if they're legit. If you put something under a crystal, it should refract and you should see double. Real crystal can easily scratch glass. Finally, real crystal is supposedly cool if you lick it, but I don't know if I can really recommend licking various crystals around Sanya.