Hi, Ronny —

The answer is, it depends.

The Rosary, as a sacramental, should be treated respectfully and not treated as a profane object. Whether it is OK to wear it as a necklace depends on the intent.

• If it is done out of piety, as a way of keeping it readily available to pray and out of a desire, say, to keep it close to the heart due to one's devotion to the Blessed Mother,
  it would not be a sin.
  
  
• If one treats it merely as a piece of common jewelry, or, God forbid, wishes merely to imitate certain pop stars who sacrilegiously wear it, or worse, wear it as a gang symbol, which apparently it is for some, that would be wrong.

Even if one wears it devoutly, (he|she) should be aware that it may give scandal. Different people have different views, and some will not approve at all.

You risk incurring someone's judgment, however unfair, by wearing it in such a fashion, so while it may not be sinful, I wouldn't recommend it.

Eric