Many Catholics, and sometimes even non Christians, wear rosary beads as if they were necklaces. This might scandalize some Catholics, who think that the person is treating the rosary in an irreverent manner.
Is it OK to wear a rosary around your neck?
The Church does not have any official declaration about wearing rosary beads. When blessed, a rosary is a sacramental, and some sacramentals (like the Brown Scapular) are worn around the neck.
However, the original saints and religious who promoted the Rosary never said what you should do with the beads when you aren't praying. Even religious have found inventive places to keep them. Dominicans, for example, wear a rosary attached to their belts.
In recent years, Rosary rings and Rosary bracelets have become popular. They are intended to be used as aids to prayer, making it easier to pray the Rosary, and not primarily as jewelry.
There is nothing wrong with wearing a rosary as a necklace, as you would a religious medal or scapular; indeed, this is a common custom in many cultures. In this way it serves as a reminder to pray the Rosary.
On the other hand, it's probably not the a good idea for Catholics to wear a rosary, especially one that is blessed, as jewelry or a fashion statement, with no intention to pray it. That is an improper use of the sacramental. Wearing a rosary in a purposefully mocking or sacrilegious way, however, or as a gang identification as some have been known to do, can be sinful.
The main thing to keep in mind is the intention of the wearer. Are you wearing a rosary around your neck as a sign of devotion and a reminder to pray, or just because it looks nice with your outfit?