I remember being told that they were not jewelry, but was never told that anything bad would happen if they were worn. That's a bit superstitious.
It seems to be common among Hispanic Catholics in this area (NC) to wear them around the neck, but I've just noticed this recently, so I don't think it was an old practice.
Even though I don't think anything terrible will happen to someone who wears them, I still don't like seeing them worn as jewelry. Just a gut reaction. I can see how it would be tempting -- some are quite beautifully made. My mother has a rosary from her wedding day that is just lovely.
Cathy Koenig