For a realistic mirror, you need to make a sheet of glass (normal glass) and put a highly reflective metal behind it.
I'm just curious, will this really produce significantly different result than just using the highly reflective metal alone?
Yes. On a real mirror, you get
two reflections, one on the front of the glass which is typically less bright, and one on the reflective metal behind it. When viewing a mirror an angle, you'll clearly see the double reflections. When applying things like dirt textures to the glass itself, you'll also see the reflection of the backside of the dirt. It's all really subtle, but getting realistic results is all about subtle stuff like that.
Also, making a double-material setup is really quite simple anyway, so you might as well do it.