I’m told that while walking in public during Shabbat, you cannot attach anything to your clothing that would not ordinarily be part of the clothing. There are women who wear their house key as a brooch pretending that it is jewelry. However, during the week, they don’t ordinarily wear this key-brooch as jewelry. And as a matter of fact it is not jewelry, it is their house key, regardless of how fancy you make it look. Is this type carrying in a public place allowed?
If the key can be used as a piece of jewelry even though it is only worn for the key – one who uses that to carry on Shabbat, can rely on the lenient opinions that permit it. (Shmirat Shabbat Kehilchata 18; 48)