Any covering as long as it is normally worn, is permitted even a comlet covering of the head and face except the eyes.

Chapter 303 Section 22

If you say before Shabbos about a coin, that you will use it from now on as a button, it is permissible to be worn on Shabbos.

Chapter 303 Section 23

It is prohibited to use a white nut as a button in a public place as people will think you are really carrying it for the nut and not as a button.