If a person has his key plated with silver or gold such that it appears like a piece of jewelry, may he then wear it on a chain or as a clip through a public domain on Shabbat? Since it is now worn as an ornament, may we consider the individual to be wearing rather than carrying the key, such that he may wear it on Shabbat?
The Shulhan Aruch (301) addresses this case and writes that one may not wear a plated key as an ornament through a public domain on Shabbat. He explains that people who see somebody wearing a key will erroneously assume that he carries it for its functional use as a key, rather than wearing an ornament. The Halachic principle of "Mar'it Ha'ayin," forbidding acting in a way that gives the appearance of a Torah violation, thus requires that one refrain from wearing a key on Shabbat. The Shulhan Aruch then cites a view among the authorities allowing one to wear a key as an ornament on Shabbat.
A famous principle establishes, "Setam Va'yesh Halacha Ke'stam," meaning, when the Shulhan Aruch plainly codifies a ruling and then mentions that others disagree, he implicitly sides with the first opinion cited. In this instance, then, the Shulhan Aruch accepts as authoritative the stringent view, which forbids wearing a key as an ornament in a public domain on Shabbat.
The Mishna Berura (commentary to the Shulhan Aruch by Rabbi Yisrael Kagan, the "Hafetz Hayim," Lithuania, 1839-1933) comments that the practice among Ashkenazim is to allow wearing a key as an ornament on Shabbat, in opposition to the Shulhan Aruch's ruling. Sepharadim, however, should follow the Shulhan Aruch's position and not walk in a public domain on Shabbat while wearing a key, even if it is worn as an ornament.
Summary: Although Ashkenazim allow wearing a key as an ornament in a public domain on Shabbat, Sepharadim should refrain from doing so.
